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.
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