Most windows use the hostname in WM_CLIENT_MACHINE, but there are windows
using the FQDN (for example libreoffice). So instead of "foo" it is
"foo.local.net" or similar. The logic so far has been unable to properly
determine whether windows with FQDN are on the local system.

In order to solve this problem the handling is split out into an own
class which stores the information of hostname and whether it is a local
machine. This is to not query multiple times. To determine whether the
Client is on the local system getaddrinfo is used for the own hostname
and the FQDN provided in WM_CLIENT_MACHINE. If one of the queried
names matches, we know that it is on the local machine. The old logic to
compare the hostname is still used and getaddrinfo is only a fallback in
case hostname does not match.

The problem with getaddrinfo is, that it accesses the network and by that
could block. To circumvent this problem the calls are moved into threads
by using QtCo...