settings: install some reasonable defaults for common languages

If ~/.local/share/gnome-builder/.defaults-installed is not found, we
will set some reasonable defaults for C, Python, Ruby, JavaScript, XML,
etc.

We will likely iterate on these defaults a bit. Especially with how they
will tie into auto-indentation in the future.

https://bugzilla.gnome.org/show_bug.cgi?id=738565