Add "Import Excel" to Data Manager.

[andrejadd]

• Get correct row range for sub setting data
• convert Sample Date
• replace single row in data.frame (date update)
• add "Convert to Excel Date" control
• Loop over sheets and detect if valid data is in the sheets.
• If more than one sheet has valid data, offer selection.
• Do not display more than N number of rows (ok, but what about new data input, with copy/paste/edit), takes long time loading rhandsontable with many rows.