Add Full image support to channels.
- ImageBackground enum has changed to ImageTheme, and the enum's values have changed as well
- TemporalField has been refactored to use generics
- Channel/ChannelGroup history elements now base their equality on start time only, to allow
de-duping of history events with the same timestamp on output.
Alter theme value to be lower-case with underscores.
Ensure that ImageTheme enum has correct values, and set a default for getTitleForDate(), so that
errors are avoided when there is no title for a given date.
Cope with nulls in equality methods for historical channel / channel group entries
Create history entries with their start date (unfortunately it can't be a private final parameter, thanks
to JAXB), so that no entries have null start date.