spreadsheet: update merged cell references when inserting rows

Fixes unidoc#212

spreadsheet: support inserting rows within a sheet

Fixes unidoc#203

spreadsheet: support inserting rows within a sheet

Fixes unidoc#203

common: handle unordered relationships correctly

Continued part of unidoc#198 after @JasonFromSpace pointed out
an error with the original fix.

meta: add go.mod

meta: add go.mod

meta: fix pdf export ole example

document: support inserting table before/after paragraph

update version

spreadsheet: add Column method to cell