Allow using selections to mask brush strokes

[askmeaboutlo0m]

Currently, selections don't affect the brush. They should mask strokes instead, like they do in other programs. This requires a protocol change though, so it's something to come in Drawpile 2.3.0 at the earliest.

We should come up with a smart way to only transmit the parts of the selection mask that are actually relevant to brush strokes. That kind of system could then also be used as a "stay within the lines" kind of masking mode in the future, since it'd work by a similar principle. Currently, selections aren't transmitted across the network at all, they're only spawned and matched locally.