Volume keys are no longer trapped by browse mode.
Specifically:
* InputGesture: add an isCharacter property which should be true if the gesture represents a typed character, as apposed to a potential command.
* KeyboardInputGesture: implement the isCharacter property. Based on the code from shouldReportAsCommand but negated, and does not contain exceptions for volume or unknown keys. space, or h, or 3 is a character, where as control+f or f2 or volumeDown is a command.
* KeyboardInputGesture.shouldReportAsCommand: a part from exceptions for volume and unknown keys, fall back to the negation of the isCharacter property.
* scriptHandler: rather than checking gesture.shouldReportAsCommand when trapping gestures in treeInterceptors, check gesture.isChracter instead.
