Add lasso selection mode to the node graph as well

Ctrl dragging in the canvas creates a lasso selection instead of a box selection. For consistency, the same should occur in the node graph.

Start by locating the canvas implementation of Ctrl-drag lasso selection, then find the corresponding selection handling in the node graph. Confirm that Ctrl-drag in the node graph currently creates a box selection, and make it use the same lasso behavior as the canvas.

索引モデルが issue の本文から書いたものです。