[FEATURE] Multiple layers merge with layer styles on Ctrl+E

This patch implements several features:

1) "Merge selected layers" is now deprecated and you can use usual Ctrl+E
   to merge multiple selection
2) Mass-merging of layers with layer styles works correctly now
3) Merging of clone layers together with their sources will not
   break Krita now.


Fixes T456
CC:kimageshop@kde.org