Implement keyboard.show for html5

[RblSb]

keyboard.show() now creates hidden focused input html element to show virtual keyboard, resends keydown/up events to game canvas and generates direct keypress events when input value updated.

canvas.onkeypress = keyPress; is removed for simplification, key presses are generated from keydown events like so:

keyboard.sendDownEvent(keyCode);
if (event.key.length == 1) {
	keyboard.sendPressEvent(event.key);
}

Using event.key instead of old event.which should be good, it's seems like a standard way to catch keypresses now and works since IE9.

I also added these small fixes while testing:

• keycodeToChar is removed (very dead code)
• MobileWebAudio._context == null check to fix ios simulator crash on first click when mobile web audio is enabled
• Do not remove all iosTouchs on pointer cancel event and do not set iosTouchs[-1] = -1; accidentally
• Explicit key.toInt() inline method, that should be useful to compare or save key values