Include user-specified row heights in saved state

This change includes a row's height in the output of `getState`, which `restoreState` will then respect. This allows users to freely resize rows and their changes will remain intact when the grid refreshes (provided that the developer makes use of `getState` and `restoreState`). Only rows that were manually resized by the user are included.

This was requested in #83.

_(also, my editor stripped some trailing whitespace and I missed it before committing; I hope you don't mind)_