User-facing entry point for in-disc menu navigation. The command
forwards an action (up/down/left/right/select/menu/title-menu/popup/
prev/mouse-move/mouse-click) to the current disc stream handler.
The new discnav input section allows binding controls that are only
active while a disc menu is on screen.
Setting the edition property on a disc jumps to the title through the
stream directly, so selecting the currently-playing edition can still
return to it from a menu.