Refactoring CellLayout into three classes

- splitting the rendering of children from the CellLayout to enhance performance, gives ~4 fps boost while scrolling on pages full of icons, and no change on pages full of widgets
- this will allow us to add hardware layer support trivially, which will increase performance while scrolling ~6-10 fps
- separated logic for caching celllayouts to bitmaps into a separate class

Change-Id: Ib6abeb19126e1504997b43c2f44af2a2fb3cd39f
6 files changed