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