Issue title: [4.0.2] previously focused window retains focus when moving to an empty space
 Issue body: With both `kwmc space -fExperimental` and OS X's built-in space movement, after I do this:

1. focus, for example, a text field in Firefox (or anything else that takes text input, like Terminal.app)
2. move to an empty space (a space with no open windows) using either `-fExperimental` or OS X's built-in space movement
3. type `spooky action`

moving back to the original space shows `spooky action` written in the originally focused item.

So far this is only a problem for me with Firefox in particular, where <kbd>command</kbd> + <kbd>N</kbd> (in place of typing `spooky action` above) opens a new window in the original space instead of the just-moved-to empty space.

Perhaps this additionally indicates a bug in Firefox (opening new windows based on the space of the currently focused window instead of the currently open space), but I imagine this is still undesired behavior for kwm.

Lastly--for what it's worth--I'm running El Capitan.

 Comments: 
 Comment 0: > move to an empty space (a space with no open windows)

There is nothing to focus on an empty space. A space can't be "focused" according to OS X, so unless there are windows populating a space kwm can't focus anything on said space. 
 Comment 1: Sorry, I somehow managed to completely overlook the fact that--without kwm--OS X automatically focuses Finder upon movement to an empty space. I'd mistakenly thought that OS X retained the focus of the last-focused application as kwm does (which I do prefer).

> There is nothing to focus on an empty space. A space can't be "focused" according to OS X, so unless there are windows populating a space kwm can't focus anything on said space.

What is OS X doing internally when you refocus an application owning no windows in the current space (and you've unchecked the system preference "When switching to an app, switch to a Space with open windows for the app")?

If I perform the repro steps as above, but I unfocus and refocus Firefox just before typing `spooky action`, then no `spooky action` actually occurs. Could this behavior be robustly emulated by kwm?

I briefly attempted the simple workaround of sending <kbd>command</kbd><kbd>tab</kbd> then its reverse (<kbd>shift</kbd>ed), but khd doesn't report successfully switching the focus (and `spooky action` still occurs).

As long as the current behavior remains, perhaps this sort of "focus-retention" should only be opt-in in kwmrc (with a warning) considering its potential for (rare) perniciousness.

 Comment 2: For what it's worth, I'm currently using this effective but mildly gross workaround:
```
# avoid Firefox focus-retention bug (when moving to empty space)
ctrl - left [Firefox]          -> : open -a Finder;\
                                    osascript -e "tell application \"Finder\" \
                                    to close front window"
ctrl - right [Firefox]         -> : open -a Finder;\
                                    osascript -e "tell application \"Finder\" \
                                    to close front window"
```
The Finder window closes before you can see it save some minor edge cases where it pops up for half a second.
{'source': 'GithubIssues', 'repo_name_with_owner': 'koekeishiya/kwm', 'issue_id': 199044669, 'repo_stars': 3937.0}