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().
Fix the 1st part of #1773: the dividing line between vertical splits
is not displayed if the split on the left contains other splits, i.e.
is not a leaf node.
