Need separate settings for indentation size and tab size
Splitting this out from #309. It's currently impossible to configure Builder to handle GLib code style. The rules in GLib are:
- Tabs align to eight spaces (editorconfig tab_width=8)
- Indentation is two spaces (editorconfig indent_size=2)
But Builder has only one setting in its preferences dialog for controlling both of these. That only works for projects where tab_width and indent_size are the same. We should enhance Builder to offer separate settings.