There was lots of fixed positioning in the Panel; now it is completely
dynamic, and width/height is driven from main.js. We still have a
global constant Panel.PANEL_HEIGHT, but this is a big step towards
eliminating it.
Also, this avoids overdraw in the "way too many tray icons" case. The
clock will shift left.
Replace 'overlay' with the more descriptive name 'overview'
where the Activities Overview is meant. Call it Overview
(capitalized) in code comments.
The overlay-group and overlay-key provided by Mutter are not
affected, since they may be used for other components than
the Activities Overview.