Fix high CPU load

* src/application-events.cpp: Invalidate UI on ItemLoad of non mail item
* src/explorer-events.cpp: Remove code to check item type

Creating and deltion of items during the check resulted in
loops of endless selectionChange events, causing high
CPU Load. Removed the code and rely on the existing no
Mail Item code in ItemLoad processing and invalidate the
UI there.

• GnuPG-Bug-Id: T7771