Currently, we listen to keyboard events to find out which keys should be represented in RawKeyboard.instance.keysPressed, but that's not sufficient to represent the physical state of the keys, since modifier keys could have been pressed when the overall app did not have keyboard focus (especially on desktop platforms).

This PR synchronizes the list of modifier keys in keysPressed with the modifier key flags that are present in the raw key event so that they can be relied upon to represent the current state of the keyboard. When synchronizing these states, we don't send any new key events, since they didn't happen when the app had keyboard focus, but if you ask "is this key down", we'll give the right answer