Camera sources: "Read current values from camera" button

[zackees]

Moved here from #1067, where @alerque suggested it and @rdoupe asked for it to go to Ideas.

Idea: in camera sources (PipeWire "Video Capture Device (PipeWire)", and V4L2 where the driver allows reading), add a Read current values from camera button that loads the camera's live control values (pan, tilt, zoom, focus, exposure, ...) into the Properties window.

Why: the saved values stay the source of truth for a set-up camera: the PipeWire source never writes saved controls on its own, and the window shows the saved value, not the camera's current one (#1067). While setting up a camera, though, it would help to pull in values that were changed from outside OBS (for example a PTZ remote), instead of the first nudge jumping back to the saved value.

Open questions

• Which control types can be read back on PipeWire (SPA_PARAM_Props enumeration) and on V4L2 (VIDIOC_G_CTRL/G_EXT_CTRLS)?
• Should reading mark the settings modified, so Cancel discards them?

@rdoupe offered to build it once it has been looked at.