Skip templates for unhandled script languages

To determine which script languages are handled, we already discover the
script engine factories, so we might just as well make use of them when
loading the templates (instead of figuring out -- again! -- which one
is the appropriate one).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a place-holder interface for syntax highlighter plugins

This will allow third-party components to provide their own syntax
highlighting plugins for additional languages.

The next commit will actually convert all of our custom syntax
highlighters into such plugins so that we 'eat our own dog food'.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Convert all of our custom syntax highlighters into plugins

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>