Fix non-working split resize with mouse drag (#1811)

Fix non-working split resize with mouse drag (#1811)

Fix the 2nd part of #1773: resize via mouse drag doesn't work if the
split on the left contains other splits, i.e. is not a leaf node.

The problem is that only leaf nodes have unique id. For non-leaf nodes
ID() returns 0. So we shouldn't search the node by id.
So replace GetMouseSplitID() with GetMouseSplitNode().