coreApp->setOrganizationDomain() expects the domain
in DNS order (hohnstaedt.de) and not reversed.
Change the WM_CLASS property to "de.hohnstaedt.xca"
via setApplicationName().
While Qt6 prepends the reversed "OrganizationDomain",
Qt5 does not and setting the complete name with
domain-prefix works for both.
Also adopt the ".desktop" file name to follow the application-name
and fix issues found by "desktop-file-validate"
