shell: replace GMenu loading code with an hardcoded list of panels

Now that we don't allow or load external panels, using libgnome-menu is just
overengineering. We can get the same results with less code by keeping a static
list of function pointers.
This reduces the number of places one needs to patch to add a new panel.
Also, this way we avoid registering all types at startup, and if we want
we can switch to load panel desktop files in a separate thread.

https://bugzilla.gnome.org/show_bug.cgi?id=690165