Add RPC support to Avogadro

This adds support for RPC to the Avogadro application. This allows
remote applications to connect and issue commands through a local
socket.

Change-Id: Ifafca6163a89818c273045c9b4507f418e66956b
Merge topic 'avogadro-rpc' into master

e203b2ad Add RPC support to Avogadro