Allow widgets to specify content type and extension

Issue #467

Signed-off-by: Michal Čihař <michal@cihar.com>
Define order of widgets to list

So far they were listed random (as dictionary does not have any defined
order), so let's list them in a prefered way.

Fixes #467

Signed-off-by: Michal Čihař <michal@cihar.com>