Use icon functions from libkleo for status icons

This avoids unnecessary and error-prone duplication of QIcon::fromTheme
calls especially if icons with new names are used where we need to
specify a fallback, e.g. data-question with fallback dialog-question.

(cherry picked from commit 7095df9af69f58dfc7def26e9208b4618069cfe8)

• GnuPG-bug-id: T7588