shell: Add device monitor for cameras

shell: Add device monitor for cameras

Pipewire allows us to easily track whether any cameras are in used by
checking the state of camera nodes. Add a simple camera monitor to the
shell, allowing us to show e.g. a status indicator.

Naturally the monitor is limited to apps using Pipewire for camera
access and thus subject to the same chicken-egg problem like the camera
portal - it could confuse users that apps may use the camera without
being noticed by the monitor. The hope and assumption here is that a
better shell integration might speed up adoption of the new camera APIs

Pipewire 0.3.49 is required for refcounted `pw_init()`/`pw_deinit()`.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2840>