- The selection service only needs to handle element selection. For
pages and sections having the index is enough.
- Those indeces are simple fields instead of observables. This also
simplifies the components which don't have to get the value out of the
observables.
- Therefore the selection service does no longer need to have a direct
reference to the unit service and it's unit.
- Selection logic is not done by the section components themselves but
by the parent canvas component.
- The unit service can now manipulate pages and section by index in
addition to passing the necessary objects directly.