[HmGui] Implement missing low level features

[HaronK]

- [ ] #236
- [x] Color structure in the engine and corresponding property
- [x] `Gui:clicked()` (can be emulated with `Gui:is_mouse_over()` + `Input:pressed(Button.MouseLeft)`)
- [x] transparent element (button) background
- [x] #223 (scrolling)
- [ ] #239
- [ ] #240
- [ ] #247
- [ ] https://github.com/Limit-Theory-Redux/ltheory/issues/251
- [ ] #252
- [ ] #253
- [ ] #254
- [ ] #260
- [ ] https://github.com/Limit-Theory-Redux/ltheory/issues/280