editor: refactor settings out of editor tab and into source view
editor: remove need for GbEditorSettings

We can just use GSettings directly. I think it results in easier to
understand code without the extra layer of abstraction.
editor: fix settings regression on new empty files

If we didn't set a language, we weren't setting defaults. This fixes that
by setting a default. We try to avoid dealing with GtkSourceBuffer at all
since we don't actually intend to ever use that.
code-assist: short circuit if there is no local file location.
code-assist: cancel pending bus operations when reinitializing

We can still have an operation in flight when we change the buffer
language (causing code-assist to reload). This saves us from having
two of those in flight at once.