If you have a keyboard layout like the following:

A B C D
 E F G
H I J K

The E and G keys here should be pressed when clicking in the empty space
next to them. This is achieved by not checking the bounding boxes of
each key and instead just using the button and row offset to extend
buttons/rows to the edges of the view. Caching for the size and
position of rows is introduced to simplify implementation and possibly
improve performance.

Fixes #191