gallery: cleanup the menubar

Remove the non-functional submenu items that belong to a manual
test application. The edit/context menu presents submenu items
well enough. Furthermore, reduce the amount of cruft in main.qml
by declaring the non-shared about action inline as a menu item
instead.

Change-Id: I1060a3ed63aa1a388e220fedddda4b7d5b412ba7
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>