Allow runtime configuration of python path.

* Added a FileBrowseWidget to simplify file testing logic.
* Changed AVOGADRO_PYTHON_INTERPRETER to AVO_... for consistency
  with other env vars.
* Make Avogadro::pythonInterpreterPath static to avoid duplicate
  symbols.

Change-Id: Ib3d37ef022ef2efcee11981d757f337f9743de03
Merge topic '154-python-exec-gui' into master

989b6dd1 Allow runtime configuration of python path.