Add a menuitem to open a new window
Rename &App menu to &Window
Remove "Claiming" mechanism

Too unintuitive to be used in a good UI. Menu items shouldn't change
their meaning.
Separate InitMadx method from frame constructor
Invert keybindings for loading model/MAD-X file
Merge branch 'static-menu-items'

Change menu items to always mean the same thing. Dynamic meaning was
more confusing than helpful.