Remove dead-code depending on sdt.core preferences.
Merge pull request #181 from dragos/refactor/remove-dead-code-prefs

Remove dead-code depending on sdt.core preferences.
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.