ToolBarManager.dispose() disposes children but does not releases them.
This is supposed to be done by removeAll(), but almost everywhere the
code was not doing that.

Changing ToolBarManager.dispose() would be a better solution, but it
looks like all subclasses of ContributionManager except MenuManager
don't do that and explicitly mention that removeAll() should be used.

Change-Id: I8f2240463616af4b00f5bef6257154ef6c528079
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>