var-type-dialog: Change entries to spin buttons, add validation.

var-type-dialog: Change entries to spin buttons, add validation.

The GtkEntry boxes used for entering and displaying width and
decimal places didn't do any validation that the width and decimal
places were valid for the selected format type, so it was possible
to enter values that couldn't actually be used.  This commit changes
the GtkEntry boxes into GtkSpinButton widgets that, as a side
effect, validate correct values through their attached
GtkAdjustments.  This is not just safer but easier to use.