Need to call the dispose on the IDocumentsHandler automatically

[codestackdev]

Currently it is a user responsibility to dispose this object. If it is not invoked from the OnDisconnect than all of the attached events are still handled when add-in is unloaded during the session.

Need to keep track of all registered documenthandlers and dispose if not disposed by the user.

To reproduce

• Create DocumentHandler
• Attach to some events (e.g. show message box on load)
• Unload the add-in
• Events continued to be capture unless Dispose is explicitly called

[codestackdev]

Fixed in 0.7.5