Cleanup shutdown and fix potential crashes

* src/explorer-events.cpp (Close): Unregister sink with addin.
* src/explorers-events.cpp (NewInstance): Register sink.
* src/gpgoladdin.cpp (GpgolAddin::~GpgolAddin): Remove shutdown
logic.
(GpgolAddin::OnConnection): Mark shutdown state as false.
(GpgolAddin::OnDisconnection): Call shutdown.
(install_explorer_sinks): Register created sinks.
(GpgolAddin::shutdown): New. Handle shutdown.
(GpgolAddin::registerExplorerSink),
(GpgolAddin::unregisterExplorerSink): Track explorer sinks.
* src/gpgoladdin.h: Update accordingly.
* src/mail.cpp (Mail::closeAllMails_o): Fix folder events
detach and do it earlier.
(Mail::installFolderEventHandler_o): Use sink instead of folder
to track event handler.
* src/windowmessages.cpp (add_explorer, remove_explorer): removed.
(gpgol_hook): Interate over actual explorers instead of variables.
* src/windowmessages.h: Update accordingly.

This fixes Explorer events release / deletion on unload / shutdown.
Previously this was only done on close, which came after GpgOL
was unloaded. Maybe this resulted in a jump to an unmapped location?

It also fixes that the gpgol_hook was only enabled for all explorers
at start time. Maybe we will see a regression now that it works
as intended on all explorers.

Explorers Events and Application Events were never deleted / released
because they were not detached before release. (But the release
then didn't delete them).

Definitely fixed is a race condition on shutdown which would be:
-> Shutdown detected
-> Close all mails
-> The currently visible mail is closed
-> Outlook loads it again because it is currently visible
-> But we are in shutdown mode.

Now we would start to parse while actually shutting down. This
could lead to hangs / crashes. The reason for this was that
we closed the mails before we detached the Application event
handler and thus captured the item load again.