[EXPLORER] Handle WM_CONTEXTMENU message in CNotifyToolbar and BN_CLICKED notificatio...

[EXPLORER] Handle WM_CONTEXTMENU message in CNotifyToolbar and BN_CLICKED notification in CSysPagerWnd.

With these, we generate the WM_CONTEXTMENU and NIN_(KEY)SELECT
shell icon notifications that applications expect when they handle
shell notification icons with uVersion >= 3.

This fixes in particular the previously unresponsive icon of KVIrc 4.x,
and more generally *all* the notifiation icons of Qt applications.
CORE-10605 #resolve