[FEATURE] Implemented Pass-through mode for Group Layers!

Now just press an icon in the layer box and you'll get photoshop-like
behavior of the group, that is the children layers will be merged as if
they were not in the group.

Still TODO:

1) Create a proper icon for the layer properties item
2) Implement loading/saving layers into Photoshop's PSD
3) Test, test and test! :)

Ref T202
CCBUG:185448
CCMAIL:kimageshop@kde.org