qmllint: Warn about duplicates in grouped properties bindings

`qmllint` implements a warning for duplicated bindings.

For example:

Item {
    property int hello: 20
    hello: 30

The second binding for `hello` will produce a warning.

Currently, this warning is implemented so that bindings to grouped properties are not warned about.

For example:

Text {
    font { pixelSize: 12 }
    font { pixelSize: 20 }
}

No warning will be issued for `font.pixelSize`, albeit it is bound twice.

Similarly in:

Text {
    font { pixelSize: 12, pixelSize: 12 }
}

Will not produce a warning albeit `pixelSize` is bound twice in the same group.

`qmllint` should extend the surface of its duplicated binding warning to cover both cases and, more generally, bindings to grouped properties.