editor: add GbEditorDocument:style-scheme-name
editor: add GbSourceView:font-name property.
settings: add new fields to editor schema
prefs: add font button to editor preferences page
editor: bridge settings to source-view/document in editor tab
source-monitor: use g_message() instead of warning.

These aren't necessarily warning type failures. Totally okay to open
a file not in a git tree.
defaults: set default for automake files
settings: rename style-scheme setting to style-scheme-name
prefs: add GbSourceStyleSchemeButton and GbSourceStyleSchemeWidget

This are convenience classes for selecting a style scheme in a more visual
way than just from a list of scheme names. This shows a bit of a preview
of the style-scheme using C as the language in the buffer.

This could probably use a bunch of work, but I think it's better than just
a list even at this point.
prefs: add style selector to preferences.
auto-indent: try to skip past strings and chars when walking backward

Not perfect, but it's an incremental start.

I still think we should try to write this differently in that we should
try to build tokens as we walk backwards, or see if we can get clang to
build us a partial translation unit for tokens.
auto-indent: try to work around escaped characters in strings.
auto-indent: add comment about future implementations.
auto-indent: skip past c89 comments when looking for matching brackets
auto-indent: use a copied iter to avoid mutation
auto-indent: add helpers to peek prev/next char
auto-indent: improve c89 comment detection