Manual Hex Input (via TextEditingController)

[tsinis]

Hey, @mchome it's me again :)

I've added an optional, nullable hexInputController parameter to ColorPicker (which I believe can help you to close Issue #31 ), instead of providing some sort of TextField or changing ColorPickerLabel to editable. So it's not a breaking change.

It allows developers to add their own custom text fields with the ability to create tailor-made validators and formatters for them, from any place of theirs app. It's simply listening to this controller and do basic validation, via two new functions. Almost every line of code is commented on and documented. Also, 122 unit-tests for those two functions were added (since TextEditingController is well tested by Google already :)) and I did not add any widgets. However, I can add some widget tests if you wish to.

I'm open to any critics, and obviously, I'm ok if you don't like this approach, so you are free to close this PR.

Thanks again for your awesome color picker, the best one out there!

Best regards,
Roman

[tsinis]

Additionally, I've removed the event type on addAllowedPointer, so it now has backward compatibility with all recent Flutter versions (uses PointerEvent at the stable channel and PointerDownEvent on beta/dev/master).

P.S.
The analyzer may complain about unknown isNot(null) in tests — just run flutter pub get