Flip Vertical and Flip Horizontal to sprites editor

[HomineLudens]

A try to add Flip Vertical and Flip Horizontal to sprites editor.

Consider it mostly like a proof of concept as some aspect of the project are still far to be clear to me : )

[pmgl]

Seems very fine to me, thank you! I think I will move the buttons to the sidebar though (I will add a main tool "Transform" and put these two options there ; they could be joined by a future rotation tool and maybe scaling tool). I am just trying to hit a first milestone on folders, then I will look into this.

[HomineLudens]

Adding it to the toolbar was my first option too, but I didn't think about grouping them, and actually is the best thing to do.
By the way, the toolbar class interface seems not so handy to allow adding line and shape drawing.
Maybe I get it wrong but I didn't found a clean way to manage mouse input to start and end the drawing, and at same time provide user feedback like the region tool.
I'll look with interest at your commits when you'll take care of the subject.
The undo management seems also a candidate point where would be nice to simplify the operation before and after a change. But even here I find hard to help and I can mostly provide only my newbie point of view.