Make it easier to subclass Content and ContentHandler subclasses

Make it easier to subclass Content and ContentHandler subclasses

Currently the names of associated Content classes are hardcoded,
meaning that any extension that wishes to subclass these implementations
must also re-implement that function, usually copying it exactly
with just the class name changed. Using "static" avoids that issue.

For ContentHandlers, I added a TextContentHandler::getContentClass,
which should be used when creating new Content objects.

Change-Id: I70f1a3291aec3460120ec20121a23f4cb68e04d1