Track cell focus and selected sheet as part of the book model

User Story

As a developer, I use Keikai to display xlsx documents.

The currently selected sheet and the currently focused cell in each sheet is part of the xlsx data.

This enable the user to return the the same sheet / cell on loading a document.

This would also allow for manipulation of the sheet selection and cell focus at Book level without having to notify UI if using operation batching.

Acceptance Criteria

move selected sheet and focused cell per sheet as property of the book model, rather than properties of the spreadsheet component.

Details

focused cell per sheet:

<selection activeCell="E11" sqref="E11"/>

https://docs.microsoft.com/en-us/dotnet/api/documentformat.openxml.spreadsheet.selection.activecell?view=openxml-2.8.1

selected sheet in book

<sheetView tabSelected="1" >...</sheetView>

https://docs.microsoft.com/en-us/dotnet/api/documentformat.openxml.spreadsheet.sheetview.tabselected?view=openxml-2.8.1