overview: Fade in desktop bar during activation

When initiating the overview effect using a touchpad, a lot of noise happens at the top screen:

1. The desktop background becomes visible (building contrast to any maximized window)
2. The outer HeapDelegate frame becomes visible (building contrast to the desktop)
3. The desktop bar becomes visible (building contrast to the rest)
4. Within the DesktopBar, the blue frame around the active desktop becomes visible (building strong contrast to the rest)

The last two points are the most egregious currently, so slowly fading in the DesktopBar while activating the window overview helps reduce the visual clutter a lot. (It was already fading when moving from the desktop grid to the window overview, so it is more consistent in that sense as well.)

Before	After