Fix issue with referrencing a non-copied video

Non-copied videos should now work, as long as it is created from a new
project.
Fix null reference exception

Happens when the description file can not be found, and therefore not
played, but tries to stop it anywyas.
Description file locations are stored set by...

relative value now.
Add UnitTest Project
Add ProjectFile factory methods, and tests for them.
Remove Use of ProjectFile constructor

Replace it with ProjectFile.FromAbsolute method.