Dropping an Element which is already visible in a Package node does not give any feedback

[gcoutable]

Steps to reproduce

1. Create a package in a general view diagram
2. Add an element in that package
3. Drag & drop the new semantic element from the Explorer View in the General View diagram

Expected behavior

A feedback message is displayed indicating the element already exists in the diagram

Actual behavior

Nothing happen

[pcdavid]

This was supposed to be fixed by #1863, I'll have a look.