modified invokation order of TextInput on_change and validation withi…

[PistachioCannoli]

…n a new private method in toga core, then updated the backends to call the new core method

I updated the invocation order of _validate() and on_change() in toga core and the appropriate backends where on_change() is validated for TextInputs. I did this by creating a new private method within the TextInput class named _value_changed and switched the call order between _validate and on_change. Then I attempted to add to existing test code to validate the _value_changed method. But I was not able to update any documentation reflecting the change.

fixes issue where validation was called before a text change was recognized, thus preventing certain fields from being properly updated.

#2325 2325

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct