Aspect ratio feature. Resolves #74

[ghost]

I added aspect ratio feature #74, but not fixed size selection. Implementation is a bit finicky when move selection is used — if cursor isn't near corner of selection and you try to move selection, it'll jump. <- Fixed in latest commit

I moved also from SlopStartMove::update clipping of rectangle to SlopEndDrag::onEnter, so selection always yields correct coordinates.

[jbirnick]

Pls add this.

[naelstrof]

I have to add it to maim too for it to work correctly. I might be able to take a look in the next week or so.

[kkartaltepe]

This is helpful without being implemented into maim (as it seems it took more than a week to carry through). For example I would like to use slop to input mappings for a drawing tablet with correct aspect ratio.

[jbirnick]

@kkartaltepe That's exactly my usecase too!

[zortax]

@naelstrof would you mind looking into this again?