Mark schema elements which are already part of the form

[sdirix]

Motivation
Help the user to quickly see which elements are already part of the form and which aren't. This is useful to avoid duplicated controls and to see for which elements of the schema there is no control yet.

Solution
Some visual cue in the schema elements tree. For example changing their opacity to 80%.

Additional context
Can be accommodated by not allowing to create duplicated controls.