Content::convert() for conv. betw. content models.

Content::convert() for conv. betw. content models.

This is needed to fix bug 41706 and similar, watch for follow-ups.

Automatic, implicit conversion may be handy in several cases, especially
for converting between different text based content models. E.g. it should
be possible to create a diff between a JavaScript and a wikitext page. This
change lais the foundations for this ability.

Change-Id: Ie7d87b67b24ac9897cb5696220a7785b228d3c79