Add supportedXXX() to Camera API

In the camera API there are actually a few classes where the camera flash support can be queried. One of those is QCameraExposure, another is QCameraFlashControl but I think there are others.

Unlike the rest of the modes like ISO, Aperture and Shutter Speed, there is no flash modes enumeration. Each flash mode must be tested separately.
I'd like an API like FlashModes fm = camera->supportedFlashModes() so that Ic an do if(camera->supportedFlashModes() & QCameraExposure::FlashOn)

Additionally, please expose this to QML, which is where I really want it.

Also, this should apply to ExposeMode and MeteringModes as well.