Handle non mail items in inbox events

* src/explorer-events: Change hasSelection to only return true if a
mailitem was selected.

Force invalidate ribon for selection to non mail items by
indicating no selection has occured.

In gpg4win-5.0.0-beta190 this is the same as
patches/gpgol-2.6.0/0002-ribbon_status_icon_fix-050508.patch