Add support for "Custom" value of "DecimalSeparator"

`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.