No export if no MESH objects, add default filename

If no MESH objects in scene then Save Dialog is not called and export is
cancelled.

Use Object names for objects in MQO rather than Mesh names.

Write file using "with" statement as recommended for Python3.

Added a default file name to Save Dialog which uses .blend file name if
exists otherwise uses first MESH object name.

Pass only MESH objects to export_mqo.export_mqo(...).

Removed filepath property from exportMQO class since unnecessary.