"Jump to" menu: Use submenus in file mode when more than one type of destinations is available.

Avoid having a crowded menu and using the old menu style.

Refs: ticket #1620.
"Jump to" menu: Separate titles and chapters in two submenus in DVD mode.

Stacking the two menus as two columns forced using the old menu style and wasn't really clear.

Refs: ticket #1620.
Refactor dynamic menus handling.

 - Various cosmetics and clean-ups.
 - More error checking in debug mode.
 - Improve the behavior of the dynamic menus when a new file is opened while a menu is opened.
Drop the "Jump to" menu and display its submenus directly in "Navigate".

Refs: ticket #1620.