add ButtonToggles

[essenciary]

@PGimenez @hhaensel it would've made more sense to be placed directly into Buttons.jl - it's the module that also contains "button group" and "button dropdown".

https://www.centercode.com/glossary/principle-of-least-surprise => Why is this button not with the other buttons?

[hhaensel]

I'll move it there then ...