Bug 881149 -  hdpi/xhdpi resolution layout for Homescreen app
     1. adjust to show 4 x 5 grid on WVGA.
     2. modify the scale ratio of app icon according to
        window.devicePixelRatio.
     3. Add 1.5x graphics (not complete, some assets are still missing)

Conflicts:
	apps/homescreen/js/grid.js
	apps/homescreen/js/page.js
	apps/homescreen/style/grid.css
	apps/homescreen/style/homescreen.css