Bug 453827 DND doesnt work for group composite

This is a architecture change for group.
Now children are added beneath gtkFrame instead of 
being added as siblings to gtkFrame.

This architecture change fixes:
 - Drag and drop for child widgets
 - Fixes Absolute positioning of child-widgets. 
    (They no longer get drawn on top of label anymore).
 - Removed background override, no longer needed with fixed hierarchy.

Tested with various scenarious:
 - Layouts (row/grid/fill/Form/Absolute)
 - Drag and Drop 
 - Drag and Drop between groups 
 - Drag and Drop in nested groups 
 - Group with various mouse events (tooltip/clicking/motion)
 - Group with Background 

Change-Id: I33898827751d5eb06460bbf94e0b6f96f03f85f5
Signed-off-by: Leo Ufimtsev <lufimtse@redhat.com>
Signed-off-by: Arun Thondapu <arunkumar.thondapu@in.ibm.com>