[WIP] support for past data in epg timeline view

As @da-anda suggested this adds support for past data in timeline view. 
It's a work in progress, because not all backends (e.g. tvheadend) supports past epg data and i want to discuss how to handle it.

Currently i use a linger timer for epg data of 6 hours, reduced by 60 minutes for the timeline view, so that you could go 5 hours back in time.

This also adds a new entry 'Go to now' into the context menu.