Adds string conversions for remaining QVideoFrame Pixel Formats

Small change that adds string conversion for the remaining QVideoFrame Pixel Formats. The determination of what format is supported shouldn't be in the purview of the utility conversion function, and adding the text for all formats there allows downstream versions that implement further formats beyond the implementation of the original FrameGrabber to use the function.

This is also a neccessary prerequisite to CameraUI taking the capabilities of the cameras frame grabber into account (which is still outstanding per the Todo note in that header).