WinGui: Redesigned the Source Selection Menu dropdown, to an overlay window. Hopefully this is a bit cleaner but feedback is welcome on the forum. This may get reverted later if it doesn't work out.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5909 b64f7644-9d1e-0410-96f1-a4d463321fa5