Fix mismatched heights on row_selectable/row_deletable columns.

[shammamah-zz]

Closes #649.

Before:

After:

[shammamah-zz]

Closing this PR as it will most likely require a more involved process to deal with the performance issues that this introduces (I suspect that the main culprit is the multiple calls to this.stylesheet.setRule, since it sets only one rule at a time.

What was changed/added in this PR:

• rows that belonged to the "fixed columns" sub-table were given indices like the non-fixed-column sub-table
• similar logic was used to adjust the row heights as was used to adjust column widths for fixed rows