budgie desktop styling #16

[fossfreedom]

This adds a remodelled Arc theming for the Budgie Desktop

Somewhat based on the default Budgie Desktop System Theme, this native Arc version has the following key highlights:

1. Slightly shaded Transparency rather than total transparency
2. Red Notification icon is now a much pleasant and less intrusive Arc selected blue color
3. More transparent MPRIS styling for Raven
4. Panel always defaults to Arc dark; GTK popups default to Arc colour styling (Arc, Arc Darker, Arc Dark)
5. Support for forced dark theming, tray stylising

[NicoHood]

Ive installed it and it seems to work. I am no budgie user, so I cannot say anything about details.

I'd merge both of your PRs now if you are happy with that. Anything else before I tag a new release?

[fossfreedom]

many thanks @NicoHood

You may want to squeeze in the sassc PR I've just done - obviously this new PR needs a slightly bit more testing since it fundamentally changes compilation from gulp to sassc. So happy to delay this PR and go with these current two PRs if you are feeling a little nervous... :)