Improve presentation in Google Sheets (in braille mode).
* An IAccessible NVDAObject with both a readonly state and editable state should remove its editable state and class itself as just readonly. This ensures that table cells with the readonly state within a contenteditable (such as Google sheets) are correctly presented and do not announce as editable text.
* Report friendly cell coordinates for spreadsheets in Firefox and Chrome similar to MS Excel, by exposing coltext and rowtext IAccessible2 attributes via the cellCoordsText property.
