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.

Closes #1257

[RblSb]

@RobDangerous i just found another reason why this PR is nice, it fixes keypress events with disabled system interventions:

Keyboard.disableSystemInterventions(Full);
Keyboard.get().notify(null, null, char -> trace(char)); // not working right now

For example, i would like to prevent default Space key from scrolling page in some browsers, but then you cannot type spaces into canvas text inputs.

I did too much verbose changes as always, so i can cleanup it to make diff more readable for review if you want.

[RobDangerous]

That would be nice, cleanups can go into a separate PR.

[RblSb]

Since there is no way to prevent page scroll on Space / some other shortcuts like /, without disabling keypress at all, i decided to use keydown for both events. But i just found that Ctrl-C will generate c char now, so i also added isPrintableText, that will check for modifiers and don't generate chars when meta/ctrl/alt is active, except when AltGr chars are typed.

KeyboardImpl is still just:

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.

And onBlur/onFocus ifs are needed to prevent System.background/foreground callbacks when this input element will change focus on keyboard.show/hide.