Moved the Mesh class from QtGui to Core

This makes it easier to use meshes with molecules, removed the
read/write locker logic and migrate over to the simple mutex wrapper.
Also move from Eigen::Vector3f to the Avogadro::Vector3f.

Change-Id: I24a4c119ed900506aa851dd8acdc056fc10731f5
Merge topic 'mesh-to-core' into master

c931b5b3 Moved the Mesh class from QtGui to Core