panelMenu: Separate from ui chrome layer

panelMenu: Separate from ui chrome layer

The chrome layer contains the user interface elements (e.g.,
the panel) that disappear when fullscreen windows get displayed.

Panel menus are currently put in the chrome layer, but don't need
to be, since they are only displayed when the user is interacting
with the shell and not a fullscreen application.

Putting panel menus in the chrome layer does mean they will get
stacked below shell interface elements that aren't in the chrome layer,
though.

This commit changes panel menus to be on the same layer as most other
shell elements, so they get properly stacked above those elements.

https://bugzilla.gnome.org/show_bug.cgi?id=657082