Issue title: Support Ubuntu Unity app indicators for minimizing to tray
 Issue body: It would be great if there was (some, I do not care which) way to have uTox under Ubuntu support minimizing to the tray, or more specifically, to support Unity's appindicator protocol.

https://unity.ubuntu.com/projects/appindicators/

I am aware that this issue is a more narrow version of #169, but as I would like to post a bounty specifically for this issue, I feel that such a narrower issue is appropriate. I would consider this closed if there is some way to have uTox running with only an app indicator visible under Ubuntu Unuty

For APIs and code examples, see also
https://wiki.ubuntu.com/DesktopExperienceTeam/ApplicationIndicators

 Comments: 
 Comment 0: [Bounty](https://www.bountysource.com/issues/7675742-support-ubuntu-unity-app-indicators-for-minimizing-to-tray)
 Comment 1: Hello mrbiber!

I'm as well an Ubuntu user (14.04 LTS) and a few months ago I googled to now if it was possible to create a kind of "watchdog" to get a DBus notification and send it to libnotify, thanks to a script.
I didn't find what I wanted, so I decided to work on the Tox project!

I'm still very interested and I'll be pleased to work on integration.
Right now, I succeeded in adding the friend's avatar next to his or her name and message when you get a notification.

I'm planning to add µTox in, the Messaging Menu. Does it sound righ to you?
Just as XChat or Thunderbird. The mesasging menu has been created to do that.

If you have any remark, do not hesitate to answer here! ;)

 Comment 2: Hi @Encrypt, sounds great! I'd be equally happy with having it as part of the messaging menu. The main point of this request is to have Tox running some way under Ubuntu where you don't need to have the main window open or in the list of active applications, i.e. like having it minimized to tray.

Does that answer the question? I've also changed the issue description and increased the bounty a bit.
 Comment 3: Hi @mrbiber !

A short message to tell you that work is in progress! ;)
What do you think about what I have done for the moment? Is it what you're waiting for?

![messaging_menu](https://cloud.githubusercontent.com/assets/9038199/5889046/df248456-a414-11e4-8ba3-ff942c28fb94.png)

Thanks in advance for your feedback!
 Comment 4: Looks great! Does that mean that µTox runs in the background, withot having a window open?

 Comment 5: pease please! add "minimise to tray" option for MATE / GNOME as well
 Comment 6: Hello you!

I have well-received your requests and I will try to implement them.

For the moment, I reviewed my code to make something cleaner (dealing with the GMainLoop and MessagingMenu library was hard to understand at first!).

A notification now disappears when the software is maximized (I mean when I click on its icon in the tray) if the display is focused on the chat box of the friend who sent the notification.

I'll add a message count on the µTox icon too in the next step of development.
Then, I'd like to add a "minimize to messaging menu" feature.

Tell me what you think about my action plan!
I'm on the #tox IRC channel if you're interested, by the way! ;)

I may commit the actual version (with the integration in the messaging menu working) before implementing the message count and minimize to Messaging Menu.
 Comment 7: Hi Yann, 

thank you for modifications and for your contribution. It's really
great job! 

However, I've updated uTox just now to version 0.18-512 and found that
there is still no tray-icon in my Mate (Gnome classic fork). Start in
Tray and Close in Tray switchers are there, but unfortunately it's looks
like in case of using Mate they cause no action. 

Dmitry 

On Thu, 12 Feb 2015 07:25:19 -0800
Yann Privé <ugonzalez@example.org> wrote:

> Hello you!
> 
> I have well-received your requests and I will try to implement them.
> 
> For the moment, I reviewed my code to make something cleaner (dealing
> with the GMainLoop and MessagingMenu library was hard to understand
> at first!).
> 
> A notification now disappears when the software is maximized (I mean
> when I click on its icon in the tray) if the display is focused on
> the chat box of the friend who sent the notification.
> 
> I'll add a message count on the µTox icon too in the next step of
> development. Then, I'd like to add a "minimize to messaging menu"
> feature.
> 
> Tell me what you think about my action plan!
> I'm on the #tox IRC channel if you're interested, by the way! ;)
> 
> I may commit the actual version (with the integration in the
> messaging menu working) before implementing the message count and
> minimize to Messaging Menu.
> 
> ---
> Reply to this email directly or view it on GitHub:
> https://github.com/notsecure/uTox/issues/758#issuecomment-74088864

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