Changing sheet in Spreadsheet editor should properly reset Operation/Edit mode
Currently, the UI doesn't reflect the internal changes when a sheet is changed in Spreadsheet editor. The result can be a view that looks like Edit mode, but is in fact Operation mode, which means that the user's changes aren't accepted. The cells are, however, editable, which looks in the user's eyes as a bug. By changing the mode to be operation for the UI as well as the internal model when changing sheets, we no longer have this issue.