Moved generation of menu VOBs from videolink_window to dvd_generator.

Fixed operation of "top menu" or "title menu" button.
Added explicit aspect ratio to menu streams encoded with ffmpeg, since some players will default to 16:9 otherwise.
Fixed off-by-one error in link coordinates passed to spumux.
Changed default padding for the body element to 10% of frame dimensions, a common rule for the title-safe area.
Added explicit assignment of actions to direction buttons since dvdauthor's policy doesn't seem to work that well.
Documented limit of 119 menus discovered by Mark Burton.
Fixed an occasional problem of links appearing in their hover state initially due to the pointer being in the middle of the virtual framebuffer.