Make VLC optional for playing sound files; use QtMultimedia as a fallback

Testing kalarm 24.08 branch I found out that now it requires libVLC to play sounds. Right now it's the only package in the whole kde requiring it as a mandatory dependency. I know that at least one distro maintainer (Slackware) would hardly include vlc and its plethora of libraries as a dependency for kalarm, and kalarm would be silently dropped for the distro. So, following the way Elisa handles this dependency, in this MR I've made the libVLC dependency optional/recommended and added a fallback to QtMultimedia.