Delay the initialization of the data container in ColumnPrivate.

Delay the initialization of the data container in ColumnPrivate until the point where the data is modified.

This heavily reduces the memory consumption in the spreadsheet when the user starts adding rows and columns in the properties explorer and intentionally or by an error enters very big numbers by appending zeros at the end of the current values which can quickly lead to out of memory situations.

This should also speed up the loading of projects with big spreadsheets since we skip the initial allocation and directly use the data containers created during the decoding of Base64 byte array.