Checkboxes cell type for spreadsheets

[juliusknorr]

After implementing form control checkboxes in #80 we will extend this to cover the cell type that represents a boolean value which will be rendered as a checkbox inline.

List of draft pull requests

• feat(sse): add Insert > Checkbox toolbar button web-apps#79
• feat(cell): add Excel-style cell checkboxes sdkjs#28
• feat(xlsx): support Excel cell checkboxes in conversion core#90