Include user-specified row heights in saved state.

Users can change a row's height, save state with `getState`, restore
the state with `restoreState`, and their height changes remain intact.
As requested by #83.
Add test for restoring resizable row height.
Add test for menu extensions positioning.
Merge pull request #171 from mminer/save-row-height-state

Include user-specified row heights in saved state
Merge pull request #174 from mminer/test-menu-extensions-position

Add test for menu extensions positioning