Make ShowGlobalSelectionMask option auto-create the mask

1) When the option is activated and there is no global mask yet,
   a new mask is created.

2) When the option is deactivated and the global selection mask
   is empty (nothing selected!), then the mask is automatically
   deleted

TODO: perhaps we don't want to save this option into KisConfig
      anymore, but somehow activate that depending on the presence
      on a global selection in the image?

BUG:343218
CC:kimageshop@kde.org