Display the distance property editor for all the anchor margins

This commit adds two features to the property preview widget :

- If a property "some.long.property" is not found in the dictionary of
supported properties, then "property" is also tried. This is useful
because some users may want to type
"some.long { property: foo, property2: bar}".
- margins, topMargin, bottomMargin, leftMargin and rightMargin are added.
Due to the first point, they match "anchors.margins" and
"anchors { topMargin: 2, bottomMargin: 4 }".

REVIEW: 116673