Add a ShellTextureCache class which loads (and can cache)
pixmap->texture conversions. This fixes a problem with the
async code in ClutterTexture that it was lower priority
than animations, and also ensures we're really only
loading these pixbufs once in the icon case.
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
