Allow for module-added compendium browser filters

[krbz999]

For example, if a module provides a lot of actors each given a flag property that can be one of several values, it would be nice for that module to also be able to add a filter in the Compendium Browser's UI to allow users to find actors that have that flag property with this specific value.

The module should need to provide a document type, a property path, and an object enumerating the keys and labels.