Put calls to Kleo::backendVersionInfo in a thread; it can be slow

Especialy on Windows systems with bad Anti Virus software this
is bad to be a blocking call in the startup routine of Kleopatra
when it is only used once the user opens the About Dialog.

(cherry picked from commit 23c2f75824500bcd071ea3851aa13579ad5faac7)

• GnuPG-Bug-Id: T6344