Issue title: Add window title to the launchers hover tooltip
 Issue body: The window list extension places the window title of a chrome app with the current website title. This is a very desirable feature to me, especially for the purpose of webapps like slimtimer.com/client, which update the title with useful information in real time.

 Comments: 
 Comment 0: Hi,

It's not clear to me what you are asking for. Could you elaborate?
On 24 Sep 2015 6:12 pm, "Bryce Schober" carljackson@example.org wrote:

> The window list extension places the window title of a chrome app with the
> current website title. This is a very desirable feature to me, especially
> for the purpose of webapps like slimtimer.com/client, which update the
> title with useful information in real time.
> 
> —
> Reply to this email directly or view it on GitHub
> https://github.com/micheleg/dash-to-dock/issues/225.

 Comment 1: With the dash-to-dock extension, it is hard to access the title of web browser tabs. With the [window-list extension](https://extensions.gnome.org/extension/25/window-list/), the title of the top-most browser tab is displayed on the list.

It would be really helpful to my use-case if the mouse-over of a running application displayed current window title next to the application name. As it is, I have to press and hold the application for a bit before I can expose the current window titles.
 Comment 2: So, you would like to replace the small tooltip which appears on hover with the windows title - the website address in the case of a browser, which already appears in the right menu, which, by the way, is accessible with the right click not only with press and hold.

One problem is the launcher might be link to multiple instances of the application/browser, in which case it is not clear what should be shown on hover. If it were the title of the topmost window it would be confusing. There is a remote possibility that I'll integrate windows thumbnails in the dock on hover, similar to windows 7 and a few shell extensions. In such case the title would appear below the windows thumbnail and it would do the job for you.

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