Add a pref for filter lanaguage.

Adds a config pref for a filter language and a language field to filters. Only filters whose language field contains the config pref appear in the filter menu.
Make a few tweaks before merging.

* Change "language" to "languages", since it's a list now.
* Don't share a single mutable list between filter instances.
Unabbreviate filter_lang.
Merge the filter_language option. Close #264.

This lets us limit the options in the Filters menu to those relevant to the codebase in question: not showing C++ filters for a Rust project, for instance. We plan to obsolete this before long with proper support for multiple languages within one codebase. Rethinking the Filters menu UI will be required before that.