Home: Use icon from AFM, display a black area with text if icon unloadable

Home: Use icon from AFM, display a black area with text if icon unloadable

* Try to find icons in the AFM_ICON_PATH.
* If none was found, use the builtin.
* If Home.qml cannot load the icon, display a shaded circle
  that approximates the size of the icons, and contains the
  icon name as text.
* Enables proper scrolling of Home icons (and clipping to its
  dimensions).

Change-Id: Ic921b357870648e675cda41985da2e985f4a0e6c