ShellAppSystem: Support loading a .desktop file directly

ShellAppSystem: Support loading a .desktop file directly

Previously, ShellAppSystem only loaded (and cached) the set of
.desktop files from applications.menu and settings.menu, using
the gnome-menus library.  The ShellAppInfo structure was
a "hidden typedef" for GMenuTreeEntry.

But we need to support loading an arbitrary .desktop file.  Thus,
refactor the ShellAppInfo into a real struct, with a refcount,
and allow it to point to either a GMenuTreeEntry or a GKeyFile.

Also, in the case where we fail to lookup an icon for an
application, ensure we return a 0 opacity texture.