* Introduce `FormService` to provide communication between
element components and form
* Add `ngModelChange` and `formControl` to
some element components. This functionality is not used by
the editor

This defines the elements priority when stacking elements on the canvas. 
Directly maps to the CSS property.

Also show tooltip on the input to explain what it is for.

The extra styling via panelClass needs CSS definition in local project, 
so can not be set by the Service itself but relies on local CSS rules.