teletextdec: add cache navigation functions

Submitted by Thomas Löwe

At the moment you have to set the page/subpage directly and you can't simply zap through the cached pages or subpages.

This patch add an "navigate" property with the values

0: show next cached page
1: show prev cached page
2: show next cached subpage
3: show prev cached subpage

to make teletextdec more user-friendly.