core: removed root menus (i.e. global menu on top of window): root menus were not really useful as most actions doable there are doable through others widgets in Cagou. For actions without equivalent in widgets (like about screen), a new menu, more discreet, will be added soon. Kivy Garden's ContextMenu is not used anymore, so it has been removed from dependencies

f95d0c31d45558c7b3e2fc39398f51eb0190bdca 0.7.0a1
4233be74572a1af8eebd502c009a10641c63ca0c 0.7.0a3
5c28cf8815319abd8c963022091cf4726764a3e6 0.7.0b1
51f73d210da908c9fb292e6bd7fda42a9d2b8453 0.7.0