js: Add nav object (navigator)

This object is tied to a navigator and can be used to ask
it to open URLs, etc
js: Improve JS OBJ <-> Showtime Property objects

We now allow enumeration and array assignments
glw: Move itemhooks before item extra options in the item menu
Improve the itemhook example

It now features a "Show details" option for video items that
opens a new page and displays details about the video (very ugly layout)