Lasso selection with the Select tool

[Keavon]

The Select tool currently lets you click-and-drag to box select a region of shapes within the rectangle. If Shift is held down before dragging, this lets the box selection add to the selection instead of replacing the existing selection.

Sometimes, users want more precise control over the region being selected than a rectangle can accommodate. For that, we'd like to have lasso selection. It should be triggered if Ctrl is held down before dragging. Unlike box selection described above where Shift has to be held down to add instead of replacing the selection, lasso selection will always add to the selection. Another difference from box selection is that the lasso must fully encompass the layer, rather than just barely touching it (as with box selection), to be included in the selection. Here's how it should work (a video mockup made using Blender):

capture_18_.mp4

The overlay system for drawing the blue lines, which is used for drawing the box selection rectangle, will also need to be used to draw the polyline of each mouse coordinate visited during the lasso selection drag. I don't think we currently have an existing function in the overlays system for drawing a polyline but it should be easy to adapt an existing function for that purpose.

[indierusty]

Implemented the method to check if subpath inside other subpath in #2183