kcm/runners: Allow simpler installation of DBus runners

While the install.sh script that is part of the python templates worked great, it still is visually annoying to have a terminal window open when we just need to copy/generate two files.

With a krunner-plugininstallerrc file, consumers can provide instructions on how to generate the dbus service and where to find the desktop file for the runner.

This is intended for python runners, but can work with any other language or even prebuild binaries. In the future, we might want to add the possibility to add dependencies. But for now, the current state should suffice.

Also, we could adjust the installation without having to edit the KRunner template and lots of existing KRunner plugins that are already published.