Update hidpi settings when it changes

[Marek Lewandowski]

devicePixelRatio is not a constant property, you can move browser between displays with a different pixel ratio.

This test case will require 2 screens witha different pixelratio.

1. Open editor in a web browser with a 1.0 devicePixelRatio.
2. Move web browser to the other, hidpi display.

Expected: Icons are updated to hdpi icons.

Actual: Icons remain blurry.

Actually if that's as simple as toggling cke_hidpi class it would be fairly simple to do that. However it will be a little harder with dev version, as it uses direct icon paths in a span.cke_button_icon.

I did not research it in deep, however there's a question on [SO that might contain interesting information](​http://stackoverflow.com/questions/28905420/window-devicepixelratio-change-listener).