add dvd main feature title detection

add dvd main feature title detection
scans the dvd menus and presses buttons to see where they might lead.
when a button press leads to a title, i check to see if it is the
longest seen thus far and save it's index.

this only applies when dvdnav is enabled.  when dvdread is in use,
the longest title of all the titles is flagged as the "main feature"

git-svn-id: svn://localhost/HandBrake/trunk@3245 b64f7644-9d1e-0410-96f1-a4d463321fa5