auto scale icon button especially for screen with high DPI

Since we don't have icon images for devices with much higher DPI, the icons look very tiny and are hard to tap on like in my Meizu MX4 which has a 480 dpi screen. It looks like this:
![s41120-105431](https://cloud.githubusercontent.com/assets/751535/5122935/52d37d88-70e1-11e4-8b94-04774810ef00.jpg)

With this patch it will look like this:
![s41120-180618](https://cloud.githubusercontent.com/assets/751535/5122943/6112487a-70e1-11e4-9ccc-95484de3a643.jpg)

And the Koreader UI on devices with 212 DPI screen is not affected. 
Revisiting this, it does not have anything to do with DPI anymore - we divide by DPI, later we multiply by DPI - so it's a NOP. This is part of my "refresh refactoring" anyway (screen.lua will go and be a part of framebuffer.lua in base), so no big issue.