Plan:

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

DONE 2. Base MainWindow and LayoutView on lay::Dispatcher

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

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

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

DONE 6. MainWindow will automatically dispatch menu_activated to LayoutView

DONE 7. GSI-expose menu_activated in LayoutView and MainWindow as "call_menu"

DONE 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! :)
   
9. BTW: make QWidget::setParent and QObject::setParent transfer ownership for
   Qt binding.

10. Provide the layer toolbox for GSI

11. Tests for new features and GSI tests

Tests:
  - compare menu layout
  - test every menu function
  - adding of new tech
  - adding of rulers/removing
  - ruby plugin (debugging, re-running)
  - changing of macro title, move macro to different place
  - funktionieren die check marks vor den menus? (Mode, selection modes, ruler template)
  - check whether there is a replacement for the "cm_..." methods and tests them (at least one)
  - check if changing the default grid works