BSF: New API base class for use with ExtJS stores

I've added a new base class for API modules that should make it easier to
implement a backend for ExtJS data stores. It incorporates most ExtJS
parameters.

This may be merged with or derived from https://gerrit.wikimedia.org/r/#/c/149845

Change-Id: I08719d3300822d2e15618b1874f4e6f6fd300328
Merge "BSF: New API base class for use with ExtJS stores"