Fix a possible crash when reactivating gpgol

* src/gpgoladdin.cpp (GpgolAddin::invalidateRibbons)
(GpgolAddin::addRibbon): New.
(g_ribbon_uis): Removed.

This keeps the ribbon ui in a member variable so that they
are cleared out when the GpgolAddin class is remvoved on
unload.

Fixes a potential crash when a user would activate gpgol
after deactivating it in the same outlook session and
stale ribbon ui pointers would be accessed.