Avoid panel relayout when opening the overview
We currently relayout the panel (and the panelGhost actor of the overview) when entering or leaving the overview. This is caused by a few things: The AppMenuButton and changing the color of the panel text/icons.
Avoid those relayouts, taking away more than one millisecond from the layout time of the first frame when opening the overview (total layout time is usually about 6 ms on my machine, without any open windows).