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
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.