Remove XMLTV TimeOffset setting and improve xmltv date parsing

The XMLTV spec requires that the grabber do one of two things:
1) Supply dates and times in GMT
2) Include a timezone/time offset in ISO8601 format with datetime
strings

Since we require grabbers and xmltv files to be XMLTV compliant the
TimeOffset setting makes no sense and is only likely to cause
mis-configurations.

The date string parsing has been improved to handling more legitimate
strings and leverages QDateTimes parsing instead of duplicating it.