Fix activation of Isolate Mode when merging the layers down
When merging layers down, a null layer is activated for a short
period of time. It happens because the following:
- When we remove the top layer, LayerBox adjusts the currently
selected index to the lower node. And tries to activate it.
- The activated node is passed further by the signals chain,
and then in KisNodeManager::slotNonUiActivatedNode() we find
out that this node has also been removed from the node graph.
Therefore we pass 0 to the lower-level code.
- Isolate mode code sees that there is no layer selected and
stops the isolated mode.
The patch fixes the issue by storing a separate flag, marking
marking the presence of isolated mode. Even when there is no
layer to isolate, we still save the sate and try to activate
it when a valid node comes.
BUG:395981