MacGui: Presets - Add abliity to modify width and height on a preset independent...

MacGui: Presets - Add abliity to modify width and height on a preset independent of the currently loaded settings via repurposing the old "Current" picture size setting in the add new preset dialogue.
- Changes "Current" to "Custom".
- When Custom is selected two text fields for width and height appear, initially populated with the current picture settings which means that if not changed will behave as old "Current" settings used to.
- Editable fields means the preset can be save to pic dimensions other than what is setup for the current encode setup.
- For example you can have a dvd loaded up and then save a preset using say 1280 x 720 as max dims, even though the currently loaded source would not have allowed that before.

git-svn-id: svn://localhost/HandBrake/trunk@3419 b64f7644-9d1e-0410-96f1-a4d463321fa5