QML ProgressBar doesn't report valueChanged events to screen reader

The progress bar does not have a value interface, so valueChanged events are not sent to the screen reader.

The progress bar should have a property that specifies whether to send valueChanged events to a screen reader.

References:

• https://github.com/qt/qtbase/blob/a1ca0c99deef5fcacb05d004ce69025886a220a4/src/gui/accessible/linux/atspiadaptor.cpp#L1013
• https://github.com/qt/qtdeclarative/blob/dev/src/quick/accessible/qaccessiblequickitem.cpp