Excel, prototype emitter: avoid the diagonal print if the style is unset (= "none", discussion #1866)

[speckyspooky]

With BIRT 4.14 the new excel emitter option of diagonals & antidiagonals was added.
The implementation is done for Spudsoft XLS & XLSX. The enhancement is not supported for the old Excle-xls ermitter.

The problem is that different teams use this emitter and with the activation of the new options diagonals will be printed, see discussion #1866.

The topic is that the style property of the diagonals won't be checked currently so the effect of unwanted diagonal print is given.

I will fix the situation that the excel-prototype will check the style configuration too.
But the emitter won't get the full support of diagonals because only the Spudsoft-emitter of excel will be supported in future.

[speckyspooky]

The fix is merged to the master branch with PR #1873