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