Prepare to support cameras

• Adds the missing bits to get the cameras identifiers in the media model.
• Figures out the passing of frame data. We stop using QImage as lingua franca, since it doesn't support some of the formats we'll need to support now (see YUV*).
• In turn, this also fixes the need for many QImage().copy() calls by properly tracking the deletion of the buffers we're sent from memfd 👍.

I suggest we merge it like this because this changes the shape of PipeWireFrame which breaks ABI.

An end-goal here is to be able to use KPipeWire to back an application like Kamoso or the pictures app in Plasma Mobile.