Add support for "Custom" value of "DecimalSeparator" #75

DecimalSeparator is now part of the MathCAT dialog. It has values of Auto, ,, ., Custom. These all work except for Custom which requires some special handling.

In MathCAT, the other values will set the preferences DecimalSeparators and BlockSeparators. when the Auto, ,, ., are chosen. For Custom, the user should be able to set them.

I think what should happen is that when Custom is set a dialog should pop up that has text fields to values for DecimalSeparators and BlockSeparators. The initial values should come from their current values.