Allow setting the homescreen grid size in Trebuchet.

The default settings for grid size cannot be changed by the user.
Our screen has a different aspect ratio than the algorithm expects
which results in too few rows.
This change allows the user to set the grid size to any values up to
10 rows by 11 columns.

Change-Id: Ie7e2b27e4744fe19d8136d6d4d495fcdd4e2ae43