allow using gradients as colors

[fmleo]

• Description:
  I know this is a bit of a really niche thing but it would be great to be able to use gradients in color fields like hyprland does.
  here's an example of what can be done with it although I'm sure most users will go for something more subtle:
  
  this could probably be applied to all existing color fields, for example window titlebars, borders, and perhaps even swaybar colors
  thanks in advance!

[ErikReider]

We'd need #25 fixed before this

[head-gardener]

This could be integrated with #97 to avoid bloating the codebase.

[ozwaldorf]

depending on how it's implemented, gradients could be animated by changing the gradient angle over time, either once when the state activates, or in an endless loop

[VehementHam]

Hyprland has this feature. I second the OP's request.

[WillPower3309]

Will be implemented once we merge in wlrfx/scenefx#65 and update the scenefx version. Aiming for 1.0

[WillPower3309]

only blocker is #367