KDEPackageAppTemplates: add "_kapptemplate" postfix to target names
Avoids chance of clashes with other user-defined target names in builds using the kde_package_app_templates macro. Also makes the purpose of the target more obvious when listing targets.
Name of internal target not specificed in the docs, so any users should also not have relied on the old naming.
Motivated by the clash seen with the executable target "kioworker" and the kapptemplate template "kioworker" in kio!1108 (merged).