Use AdjustingScrollArea in key approval dialog

This fixes the resize in NewKeyApprovalDialog to work as
intended and adjust the size within the limits defined there
since the AdjustingScrollArea reports a sizeHint that is
related to its contents.

(cherry picked from commit 31ee4f9790c14f87d2daeb0aa9af335795e6c9e3)

• GnuPG-Bug-Id: T6837