Gnome-style window controls appearance (hover and sunken effects)

[ripefig]

In Adwaita, there is a very subtle highlight around shapes in the headerbar, which gives them a sunken effect. This is most obvious with the magnifying glass in the gif.

[kupiqu]

This may be a good opportunity for a (Gnome-friendly) person that wants to contribute to Sierra Breeze Enhanced.

If somebody is interested I could help to get started...

[kupiqu]

Closing for lack of interest. I'm open to review a pull request

[kupiqu]

Reopening this but keeping it low priority

[kupiqu]

Button padding is implemented but this feature actually relies on (1) within-button symbol padding, and (2) rounded rectangles instead of circles (which should be applied specifically to the gnome style), only then it would make sense to replicate (3) the changes in color and gradients.

(2) and (3) seem relatively straightforward, (1) may be more tricky to accomplish.

EDIT: Well, for (1) I can actually make bigger default buttons for gnome keeping the same symbol size...