Bug 520685 - [Generic Editor] Sort ext. by most specialized content-type

Factorize extensions and registry polling (can be continued).
This allows most registries to return first the extension that applies
to the most specialized content-types.

Change-Id: Icfd5d0c2e72cd579e9fe8c034420d88c628f39a6
Signed-off-by: Mickael Istria <mistria@redhat.com>
16 files changed