Add option to use system GTK theme titlebar icons instead of bundled Adawita set (Zen 1.14b)

[rotntake]

As of Zen 1.14b, the titlebar icons were overridden with what I'm pretty sure are the titlebar icons from the Adwaita theme. I'm guessing this change was made to allow making the color of the icons adapt based on dark and light modes.

Now custom GTK themes are sort of a niche thing and not everyone uses a custom GTK theme, so it'd be understandable why it might be pointless to have such an option. But still I'd love having theme consistency with my other apps.