MacGui: improved the presets management
Added a class to manage the presets (HBPresetsManager) and moved the presets drawer code to a new view controller (HBPresetsViewController).
Removed the limitation of two nested folders and added a way to add to custom folders.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6273 b64f7644-9d1e-0410-96f1-a4d463321fa5