[dvbviewer] get rid of external XML parser library

Get rid of external XML parser and use the already shipped tinyxml
library. This also speeds up parsing of large channel lists by an
order of magnitude. Additional we can remove some helper methods.
[dvbviewer] don't fetch epg language any more

RS 1.26.0 will provide us with already merged epg data: "Now the correct
language of the EIT data is filtered while parsing and only this data is
saved".
Also bump required RS version
[dvbviewer] implement low performance mode

this mode is intended for low performance devices like the raspberry pi.
for now this only disables logos+thumbnails
[dvbviewer] remove more of the hungarian notations
[dvbviewer] remove even more of the hungarian notations
[dvbviewer] reduce implicit calls to SwitchChannel

thanks to greco
[dvbviewer] tsbuffer: remove hungarian notations
[dvbviewer] don't use timezone offset for timers
[dvbviewer] pump version to 1.9.14
Merge pull request #290 from manuelm/master-pull

dvbviewer: general improvements