Trebuchet: Adjust the grid size screen layout for sw600dp

Comfortable - before: http://i.imgur.com/yZ1cM9q after: http://i.imgur.com/DQNsEfD
Cozy        - before: http://i.imgur.com/EeAgygS after: http://i.imgur.com/T7Nuauz
Condensed   - before: http://i.imgur.com/b2nTUxJ after: http://i.imgur.com/ZoAI4gn
Custom:     - before: http://i.imgur.com/H2rrvr3 after: http://i.imgur.com/RSQxwfQ

PS2: Makes dynamic_grid_size_image overridable
PS3: Fix a typo in the commit message

Change-Id: Ic6b68953758a691cd032ba27030fa7a688efb293