style-schemes: use the style scheme for diagnostics underlining

Builder underlines segments of code in different colors depending on the
condition reported by libclang (deprecated, error, note, or warning). As
of v3.17.2, GtkSourceView supports the underline-color attribute in
style schemes. So Builder now makes use of that, and falls back on hard-coded
values in case our custom style scheme isn't installed.

https://bugzilla.gnome.org/show_bug.cgi?id=767216