popover: Fix submenu navigation

popover: Fix submenu navigation

In commit 024d832d943b2abf4f0d, we introduced a
cascade-popdown property that makes closing a submenu
propagate up and close its parent menus. This is the
behavior we want when a menuitem in the submen is
activated.

What we overlooked is that we still need to be able to
close a submenu during navigation, before opening another
one. And in this case, propagating the closing is breaking
things. Fix this by adding a private close_submenu api
to GtkPopoverMenu that avoids the propagation.

Fixes: #3301