qmlls: completion for property definitions with bindings

Implement completion for property definitions. The problem here is that the FileLocations of the Dom overlap in the property definition:

property int x: 42

The green part is the property definition, the blue part is the binding and the red part is overlapped by the binding and the property definition. The DomItem you get for the red part is the Binding, but from the Binding there is currently no way to find out if this Binding is inside a property definition or not.

The main difference between completions in bindings and completions in property definitions is that you should not complete a property definition (it should be a new name chosen by the user, the completion of the 'property' keyword is currently working) but that you should propose existing names for the binding completion (when outside the property definition).

Find a way to differentiate both case in the Dom or fix the FileLocations to stop overlapping each other.