navigation: add support for multiple decoders
epgcache, lookupEvent: clear eServiceEvent pointer

In several occasions, the pointer is reused by
the user, and contains the previous result.
When the retval is not checked, this leads to
duplicate events being displayed.

So make sure the eServiceEvent pointer is NULL
when we return an error.
vtuner: do not start a pes filter untill feeds are requested

dvb-usb-v2 no longer supports feeds to be started while
the frontend is sleeping (adap->active_fe is set to -1).
Not only does it not allow this anymore, but trying to do so
seems to trigger bugs, when adap->active_fe is used as an
index in adap->fe[]

So we wait till the vtuner device requests feeds to be
started, by that time the frontend has been activated
and tuned.
servicemp3: different handling of subtitles from gstreamer.

- cache all (future and past) subtitles received from gstreamer,
not only "current" ones, this works better, because gstreamer
does not always send subtitles in time for enigma;
- use map instead of list for cache for better efficiency;
- use locking on the subtitles cache data;
- don't throw away subtitle cache when seeking;
- store only necessary information (start, end, text) in cache,
instead of complete PangeSubtitle structure per subtitle;
- use ns/ms/pts units more consistently;
- use subtitle end time instead of start time to decide whether
subtitle is current and take already passed time since begin of
subtitle into account, this enables displaying subtitles that
are "skipped into half way" when step-skipping.

Thanks to pieterg and sjaaky for their input.
servicemp3: remove unnecessary locking for subtitles.

It appears the code accessing the subtitles cache is run
all within one thread and the enigma2 timer mechanism is
non-preemptive.
esubtitle.cpp: cosmetics and small optimisations
esubtitle.cpp: honour "rewrap subtitles" option for "gstreamer" media
text subtitles
servicemp3: subtitles: reduce debugging output and remove an unused variable
Merge branch 'master' of git://git.code.sf.net/p/openpli/enigma2