Fix visible windows report for linux

[exekias]

Map State flag is IsViewable when the window is visible to the user

[exekias]

this doesn't seem to work with Unity

[seletskiy]

@exekias: But it works for i3, so, maybe, add WM detection?

[sixstone-qq]

@exekias It seems it is only failing for two different firefox windows in different virtual desktops. I don't know why, because Map State: IsViewable is always set, no matter which window is but the visible ones only includes the ones in the same virtual desktop the active window is and that firefox window.

[beyang]

@exekias Could you submit the patch as a separate implementation of the Tracker interface? See my comments here: #13 (comment)

[beyang]

Following up here: I tested this with Unity and indeed this approach does not work. In Unity, all windows listed by wmctrl -l seem to have Map State: IsViewable.

However, if this is a valid approach for i3 and other window managers, could someone refactor the change to be a different Tracker implementation and then use that implementation when the appropriate window manager is detected?

This should be a very simple change, but I am not the one to do it, because I do not use i3 or AwesomeWM, and so wouldn't be able to test it.