fix: move extension properties to preferences

[benoitf]

What does this PR do?

Make preferences visible (and only the one with the default scope) when adding simple properties to an extension.

Screenshot/screencast of this PR

What issues does this PR fix or reference?

#947

How to test this PR?

Add some properties (like in podman package.json file)

      "properties": {
        "podman.my.test.property": {
          "type": "string",
          "default": "test",
          "description": "My test property"
        },

and look if you see them appear in the preferences screen.

Change-Id: If8889eae5c1ef13232d22fe2539b941b728e2fef
Signed-off-by: Florent Benoit fbenoit@redhat.com