cinnamon styles: add option for user to save current setting as a new user style

[anaximeno]

Maybe we can add a button somewhere in this section for users to save their theming configuration as a user style, and it could open a window dialog (or something like that) requesting the user for the name of the style and determine the variants in some way:

After that, the style could simply appear on the select button of the simplified settings:

There would be necessary something for users to remove and update the styles they created as well, so maybe a style editor window would be necessary. Anyway, I don't feel like this is a super-required feature right now but just wanted to record it here in case we want to implement it in the future.