MacGui: fix loading preset which specify an advanced options string (rather than using the x264 preset system).

Also miscellaneous cleanup and improvement.

The issue was introduced in SVN revision 5388.



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5392 b64f7644-9d1e-0410-96f1-a4d463321fa5