Numeric Editbox

The typical MATLAB uicontrol edit box allows for any string to be input
by the user. Often times, we want numeric values only and have to safe
guard against this in the edit box callback:

function editbox_callback(src,evt)
if isnan(str2double(src.Value))
warning('Not a string')
src.Value = 1
end

This class saves the effort of checking this and provides a few
additional features as well.

In addition to the standard uicontrol properties used by an edit box, there are
four additional ones:

* Value: The numeric value of the edit box.
* RoundFractionalValues: If true, the numericEditbox only accepts integers.
* Limits: Minimum and Maximum acceptable values.
* ValueChangedFcn: Callback that fires when a valid new value is accepted.
There is also a 'ValueChanged' event that can be listened for when an
acceptable input changes the 'Value'.