All tools now respect selection

The selection made in the select tool now affects the other tools (brush, paint bucket, shape, gradient, text). So for example you can limit the area of your brush strokes without doing tricks with alpha lock.

There are still some problems to be solved:

• Drawing with the pixel brush in a selection can lead to anti-aliased edges. It should always be pixelated.
• The paint bucket flood fill algorithm should be limited by the selection. Currently, a mask is simply applied after the flood fill.
• Smudge brush doesn't always fully reach the corners of a selection.
• The edges of the selected area look pixelated when drawing a stroke that goes back and forth in that area. I would need to implement performance improvements before I can fix that. That's more of a long term goal.
• The clipboard dialog should do something with the selection too.

The implementation of this update was much easier than I initially expected, so it felt a bit anticlimactic to be releasing it. But I think it's a big deal to have the selection basically fully integrated.

-bitbof

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