Added the VFSSource to the python binding.
Changed the VFS::addNewSource method to return the created source.
Added the VFS::removeSource method to the python binding.
The source providers now store sources they already have created.
Added hasPath methods to all source providers, this returns whether they already hava a source for a path.
Added hasPath method to the VFS, this will return whether it has an active source for that path.
The VFS no longer needs to keep track of a already paths itself.
Fixes #835