Fixed a bug in linking two flycameras
Fixed a bug in linking two flycameras

See issue #1554. The bug is fixed as discussed.
- Replaced actual `rotation.setter` function by a warning of using `rotation1.setter instead`
- Added property `rotation1` and `rotation1.setter`
- Added property `rotation2` and `rotation2.setter`
- Normalizes Quaternion before assigning.
Eric89GXL/vispy,Eric89GXL/vispy,Eric89GXL/vispy