cantata: make qtmultimedia and vlc optional

Upstream defaults vlc to off. Its only purpose is as an alternative to
QtMultimedia for "MPD HTTP stream playback" [1]. Similarly, provide a
toggle for qtmultimedia, in case somebody wants to disable HTTP stream
playback.

[1] https://github.com/CDrummond/cantata/blob/efa907c8e03d052718f14834eb968da86d1c34d8/CMakeLists.txt#L51