MAGN-5461 Add an event for Workspace closed.

[MAGN-5461](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5461) was originally created to add events for the "closing" of a workspace, at the request of the Robot team. As it turns out, we've got two events, WorkspaceClearing and WorkspaceCleared which already achieve the aim of this pull request. This pull request therefore does the following:

- Renames the CleanWorkbench method (from back when I called it a workbench) to ClearWorkspace (which is actually what the method does).
- Moves the WorkspaceClearing and WorkspaceCleared event triggers inside the ClearWorkspace method to ensure that they are always called. There are several places where this method is called and the events were not always being triggered.

PTAL:
- [ ] @pboyer