Add an option to run full KDE or just kwin

[skipperro]

When you want to stream just a single application, without desktop environment, this application won't be resizing to fit browser window, because this feature depends on window manager. This is a solution:
If WM_ONLY variable is true, entrypoint will run just kwin (window manager) instead of full KDE session to support application resizing without giving access to whole desktop.

Otherwise everything will be running as usual.

[ehfd]

I think a better way is an environment variable specifying the startup program or script in the container. It should be able to work more universally. I will implement when I have time.