MacGui: added the list of the presets at the bottom of the preset menu and a “New Folder” menu item. Removed the “delete built-in presets” item because it takes just two clicks to remove them manually.
Refactored part of HBPreset to a separate HBTreeNode class.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6278 b64f7644-9d1e-0410-96f1-a4d463321fa5