Add string context and location filters to the translate interface

Currently we can filter by string, but not by context.

With a context filter you can easily list, for example, all translatable strings in the "Node types" group for some node bundle (e.g. page) by using the "type:page:" context filter.

The use-case which actually prompted this was using the webform localization contrib module, which incorporates the webform node ID into the context of each translatable string. It's really useful to be able to list strings for a specific webform, and omit the others, as things get really crowded really quickly.