Tab names are not allowed to start with a number

[per1234]

4545283 made it so that sketch names starting with a number are now allowed but New Tab still doesn't allow a number to be entered as the first character of the tab name.

Originally reported at: http://forum.arduino.cc/index.php?topic=496959

[cmaglie]

Thanks, now is fixed.

BTW I'm not fully satisfied with the solution since JTextBox implements his own validation logic, this must be refactored to use the BaseNoGui.sanitizedName() method to validate the input.