Add flip_h and flip_v to Button icon

[acgc99]

This will reduce the use of rotated icons (separateed image files) if required. It would work like TextureRect.

[Mickeon]

I do not feel much interest for this. What's the use case?

[acgc99]

Just giving the option for rotating the icon would reduce the number of icon files. The main use would be for arrows I suppose.

[samuelfine]

Checking in a year after this original request - I could definitely use this, specifically for the reason given here. It will allow for quick reuse of textures as Button icons without requiring additional unique assets, similar to how it is used in Sprites today.