WinGui: Add a new command line argument to the HandBrake GUI  executable (--reset). This deletes all preset, user preset and settings file which should result in a full reset to defaults. 

Also fixed a small bug on the Add to queue button. Don't actually add an item without a destination.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6091 b64f7644-9d1e-0410-96f1-a4d463321fa5