Drag and drop was not working since the (non-reactive, invisible)
chrome group was on top of most of the screen. Explicitly set
a 0x0 size on that group to resolve the problem.
Adds an explicit "chrome" layer for the panel (and later the sidebar),
managing the input region and struts for them, and hiding them when
fullscreen windows are present or the user enters the overlay.
http://bugzilla.gnome.org/show_bug.cgi?id=581771