FEATURE: Quick Ungroup action (Ctrl+Alt+G)

1) If a group is selected, all the child layers are ungrouped and
   the group itself is removed.

2) If all the child layers are selected, they get ungrouped and
   the group itself is removed.

3) If only some child layers are selected, they just get ungrouped.
   Their parent is kept alive.

CC:kimageshop@kde.org