DVB playback experience improvements

Hi, 

What I've done
- [x] Don't update signal statistic when channel is not active to avoid showing dummy numbers.
- [x] Show time of current event in status bar
- [x] Add `m_DVBState` to store various informations about currently watched channel needed to store start/end time of the event, but it was adopted for many other things too. 
- [x] Mpeg2SectionData: Stop parsing data as soon as we know this is not what we seek for. 
- [x] `TIMER_STREAMPOSPOLLER(2)` mostly cosmetics to avoid code duplication and wrap everything into switches. 
- [x] `TIMER_STATS` don't merge current and average numbers for bit-rate when they are equal. It looks stupid, because for constant bit-rate content, like audio it just switching between showing one or two numbers. There are always some deviations and it will show like `641/640 Kb/s` or `640 Kb/s` randomly. After my change second case will be `640/640 Kb/s` always.
- [x] Fix event time calculations. They were one day off in certain cases. Needed for properly displaying event time in the status bar.
- [x] Add channel name to filename when saving snapshot. 
- [x] During scan accept only supported services. This is needed to filter out private data services we don't support.
- [x] Context menu with basic operation of reordering and sorting list and removing channels. Store channels in std::vector for easier data handling. 
- [x] Sync translations (manual push to transifex will be needed)