Make the bottom borders of reorderable tabs not look like trapezoids

The junctions between borders are rendered slanted by CSS. Replaced left and right borders by inset shadows.

https://bugzilla.gnome.org/show_bug.cgi?id=729465
Make symbolic icons work with the current rsvg

The rsvg loader now restricts what external files it will
allow to load from an svg. Thus our xinclude trick doesn't work
anymore. To work around that, embed the payload in a  data: uri.
This is somewhat ugly, but it is what GTK+ does.
HC: provide legacy gtk stock icons

https://bugzilla.gnome.org/show_bug.cgi?id=668074
HC: gnome icons are adwaita now
render from adwaita

- adwaita provides multimedia-volume-control now

https://bugzilla.gnome.org/show_bug.cgi?id=708311
Improve the look of ON switch

The through is now flatter, and the slider's border is now transparent black instead of gray, so it doesn't look odd on blue background.

https://bugzilla.gnome.org/show_bug.cgi?id=729558
There were unused hover states for checkboxes and radios in the assets - use them

https://bugzilla.gnome.org/show_bug.cgi?id=729561
Make GTK+ window shadows more consistent with Mutter shadows

True uniformity is not possible yet because GTK+ uses a different blurring algorithm, but this looks good enough.

https://bugzilla.gnome.org/show_bug.cgi?id=729574
Make window buttons look like normal buttons

https://bugzilla.gnome.org/show_bug.cgi?id=726006
Fix infobar backdrop state
Use Tango color for theme selection color
Base selection bar colors on base selection color
Make suggested action buttons relative to selection color
Fix typo in render script
render missing assets.

https://bugzilla.gnome.org/show_bug.cgi?id=729587
Elegant recolorable borders

That don't rely on hardcoded colors in assets.
Don't use assets for backdrop borders
Fix level bar troughs
Remove trough border asset usage
Don't use border assets for primary toolbars