Warn the user when deleting keys that are part of a keygroup

This is deliberately not implemented in the form of an additional warning dialog,
as there are already a few dialogs when deleting keys, increasing the risk that
the user just ignores all of them entirely.

(cherry picked from commit 73b57a8c8030991d13ff822d86ec8d2dafd5db91)

• GnuPG-Bug-ID: T6403