Improve selection mask display

[askmeaboutlo0m]

Currently, selection masks are previewed in a light blue color on top of the canvas, similar to how SAI does it. While that works okay, it can have visibility problems when you're dealing with blue colors. Some considerations to that effect.

We could let the user pick a color through a menu, probably at the top-left of the dock, similar to where there is one in the fill tool. This is reasonably easy, fast and likely good enough for most cases.

We could also change the blending on this, using something similar to the brush outline. Problem with that is that the only blend modes that do this kinda thing are Difference, Exclusion and (for software rendering) XOR, which are all so ugly that you wouldn't wanna look at them.

Otherwise, instead of having the selection mask rendered as a Qt graphics item on top of the canvas, we could go through the canvas rendering proper and use some kind of more intelligent blending that makes sure it's visible. However, this is kinda slow, harder to implement and finding a blending algorithm that manages to remain visible while not being horribly ugly is gonna be tricky.

Regardless of that, we could invert the mask so that the outside is what's changed, rather than the inside, which might be better visible. It's also how Krita displays it. This was suggested by SadColor.