[NewChild] Provide on the ModelExplorer a new creation menu

The goal of this task is add a new creation menu in the ModelExplorer.

The current menu	the current dialog

Papyrus-Web already provide a creation menu in the ModelExplorer (see upper images). This menu opens a dialog where the available elements are grouped by containment feature. It is not userfriendly, it takes time for the user to find the element he wants to create.

We propose to create a new action, called "New Element" in the ModelExplorer. This action will open the same dialog than the dialog already proposed in the Diagram. This dialog already provides these features:

• a search field
• new Node menu
• new Edge menu
• last created element

(snapshot is coming soon)

At the end of the process, the current "New object" menu should be hidden/deleted from Papyrus-Web.