Handles follow moved selection and resize on rotate, and...

[PurelyAndy]

constraining with shift preserves aspect ratio.

[cameronwhite]

It's been a while since I looked at this branch so I'll need to refresh my memory on what my thoughts were for future changes :)
I think I wanted to use Cairo.Matrix.GetComponents() to decompose the transform into its rotation / scale components and then rotate the handle along with the rotation

One other note is that the selection tools also use the rectangle handle, so having the ability to constrain to a square selection is still needed for the current behaviour (but probably can share most of the code since the aspect ratio is then just 1:1)

[PurelyAndy]

I have no idea what I'm doing. Pushing the unfinished changes I made months ago. Maybe someone else will be able to build off of my work in the future.

[cameronwhite]

Thanks for the update! I'll close this but it'll be kept around for future reference.