Fix the "panel" icon to be symbolic. Make the overview parts only show
up when in the overview, and the non-overview parts (eg, the Desktop
window, if there is one) only show up when not in the overview. Sort
the different items consistently with their locations on the screen.
https://bugzilla.gnome.org/show_bug.cgi?id=618887
CtrlAltTabPopup was using a St.BoxLayout and relied on anchor_gravity
center for positioning. This does not guarantee correct pixel alignment,
so use a St.GenericContainer instead and do the positioning similar to
that of the appSwitcher.
https://bugzilla.gnome.org/show_bug.cgi?id=643820
