Multilayer selection: implement drag & drop of multiple layers

This changes KisMimeData to handle a list of layers. If you try to drag a
complex selection with groups and so on, it will check whether the group
that contains a selected layer is selected and then drag the group; otherwise
it will drag all the layers in a flat list, instead of trying to keep
the group hierarchy with the unselected layers discarded.

This is the behaviour that Photoshop shows for me as well.