Add support for dragging UIAction objects on UIPackage elements so that
the package that an action belongs to can be changed. This functionality
is not complete yet, since we can't yet undo the operation. Also, the
ActionEditor does not yet refresh automatically.
Very simplistic layout algorithm for placing UIAction
on Package Diagrams.
Refresh the content of the ActionEditor if an action is moved into a new
package.
Add listener notifications for when an action's package changes. The
notification is actually to inform the listener that the package
membership has changed.
Detect when the underlying model (for a package) has changed, and update
the package diagram when necessary. This usually happens when the model
is changed by one editor, and the other editor tabs need to be updated
accordingly when they get focus.