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.