Fixes and cleaning for KisLayerBox

This patch fixes:
     1) Drag-and-drop (it has been broken recently)
     2) KisNodeManager::activeNode() is now set properly, when a node
        is deleted (caused a crash).
     3) Controls of KisLayerBox are enabled/disabled properly again
       (has been broken as well as 1) )

Does *not* fix:
     1) Crash when adding a transparency mask with a precreated
        selection (bug 266687)

CCMAIL:kimageshop@kde.org