Plan:

1. Combine AbstractMenuProvider, PluginRoot and the AbstractMenuProvider's 
   implementation in MainWindow/LayoutView into lay::Dispatcher

2. Base MainWindow and LayoutView on lay::Dispatcher

3. lay::Plugin::plugin_root -> lay::Dispatcher *lay::Plugin::dispatcher()
   This method will find the MainWindow or LayoutView depending 
   which is the root.

4. Don't use AbstractMenuProvider::instance anymore, rather use lay::Dispatcher::instance
   when necessary (should not be)

5. Instead of slots, bind abstract menu items to symbols passed to 
   Dispatcher::menu_activated

6. MainWindow will automatically dispatch menu_activated to LayoutView

7. GSI-expose menu_activated in LayoutView and MainWindow

8. We can move menu initialization of LV-specifics to the LayoutView and
   remove the cm_... and the code which just passes signals to the current view! :)