Add quad symmetry (horizontal/vertical at the same time)

[dacap]

Source: https://twitter.com/TheActualRiz/status/751393552924565508/photo/1

Related to #1171 to improve the symmetry selection UI.

[SolarLune]

Hey, I just popped up to ask this, too. You could just not have the existing symmetry buttons cancel each other out, rather than adding another button (in case that was how you were thinking of doing it).

[thezakman]

Will this be added?

[dacap]

@thezakman yes, this will be available on v1.2.