1) The button "Line header" becomes a split button with a menu to choose between 0, 1, 2... 4 line of header + a menu with a dialog box to choose any number of headers.

2) Merging headers and non headers cells will be forbiden.
(so changing the number of line of headers can be prohibited due to merged cells)

3) Authorize lines insertion even in headers (that will produce new header lines)

+ the same on cols