Adds real content check to the content describer

To avoid the cases when play template editor code is used inside other
editors.
Fixes #176
Adds new template editor to HTML files

Adds the new template editor to the 'open with' submenu, for an easier
access in case the content type was incorrectly computed.
Merge pull request #182 from skyluc/issue/leaking-in-other-editors-176

Adds real content check to the content describer