Add DisabledReason field to MenuItem

This is useful to disable items that are not applicable right now because of
some condition (e.g. the "delete branch" menu item when the currently
checked-out branch is selected).

When a DisabledReason is set on a menu item, we
- show it in a tooltip (below the regular tooltip of the item, if it has one)
- strike through the item's key, if it has one
- show an error message with the DisabledReason if the user tries to invoke the
  command

pkg/gui/controllers/menu_controller.go

@@ -54,7 +54,7 @@ func (self *MenuController) GetOnFocus() func(types.OnFocusOpts) error {
 	return func(types.OnFocusOpts) error {
 		selectedMenuItem := self.context().GetSelected()
 		if selectedMenuItem != nil {
-			self.c.Views().Tooltip.SetContent(selectedMenuItem.Tooltip)
+			self.c.Views().Tooltip.SetContent(self.c.Helpers().Confirmation.TooltipForMenuItem(selectedMenuItem))
 		}
 		return nil
 	}