when using native gtk3 menubars.

The issue is that MenuBarManager does not own its MenuBar.
And in this embedded menubar situation a new menubar is newed and passed to
m_pInplaceMenuBar but nothing destroys it.

Now with native gtk3 menubars this becomes obvious as the native menubar stays
behind, while in the non-native case the old menubar is replaced by
the new one so while it still leaks the menubar you don't see it.

Change-Id: Id732cb66664a71efc471d7bad35f4de890e1017e