Now the mapping between KisNodeDummy and QModelIndex is done by
classes KisModeIndexConverter and KisModelIndexConverterShowAll.
They represent two options available to the user: normal mode and
mode when the root layer is visible.

This is the last commit in a series of refactorings for KisNodeModel.
Now there should be no known crashes or bugs. So if you see one, do
not hesitate to report! ;)

My plan is the following:
1) I'm pushing it to my branch.
2) Then I'll fix drag-and-drop (it is unrelated to the refactorings)
3) Then I'll make a call for testing both of the features, to
   save the time of artists ;)

BUG:291271
BUG:290708