Improve number input dialogs #321

2.7's ScaleMenu and #318's IntRangeConfigMenuItem both use JOptionPane.showInputDialog(...) to create a dialog prompting the user for a number.

The dialog accepts any string and gives no feedback when the input is invalid.

It would be much more user-friendly to create custom dialogs that don't allow submitting invalid input and give feedback on what's invalid.

Polish could include:

• outlining the input field in red when it's invalid
• displaying a red message explaining why the input is invalid
  • not a [whole] number
  • not in range
• step buttons