GWL: Fix thumbnail preview issue when window fade-out setting is non-zero

[ronyala]

In order to have fade-out animations, multiple preview overlays are needed at the same time.
The existing code did not handle this, instead not creating a new overlay and not showing a window preview if an existing overlay was fading out.

I did not find any existing issues that were reported about this.
I also did some related code cleanup.

[clefebvre]

@ronyala can you describe how to reproduce the issue?

[ronyala]

@clefebvre

1. Set the window fade-out time in grouped-window-list to a value greater than 0 (Ex. 150ms).
2. Open multiple windows of the same program, it's most apparent with at least 4-5 windows
3. Hide the existing windows behind a different program so it's easier to see (Ex. multiple nemo windows hidden behind a firefox window)
4. Hover over the window thumbnails in gwl, at least half of them will not show the corresponding window (new overlays/previews are not created while there is an existing overlay)

[clefebvre]

Perfect, I can reproduce both the issue and the fix.