Draft: plugins/screencast: Make PipeWireCore persistent
PipeWireCore is created on demand. However, it fails to initialize after updating libpipewire.
It looks like libpipewire loads some plugins. The theory is that the libpipewire version is burned in the lib and the plugins. After updating libpipewire, kwin's libpipewire and pipewire plugins have mismatching versions, and so pipewire encounters unexpected case, which results in a crash.
To work around this issue, this change makes kwin keep the event loop and context objects even if there are no streams active anymore.
Draft: no idea if it's correct, but kwin no longer crashes after updating pipewire