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
Merge "Make it easier to subclass Content and ContentHandler subclasses"