Fix flatten/merge down/merge multiple action handle locked groups correctly

This patch extends the already existing strategy, which was previously
used for merge down operation only:

1) If there is at least one unlocked layer in the selection, then the
   merge will happen.
2) The merge result will be written into nearest unlocked parent
3) All unlocked layers of the selection will be removed
4) All locked layers will be kept in the layers stack, but switched
   into *invisible* state.

CCBUG:406697
CC:kimageshop@kde.org