Add '@objc' to ChartViewPortHandler

[AlBirdie]

For custom touch processing (i.e. a UIView with several charts in it that will receive touch events from this UIView), we need to expose the ChartViewPortHandler to Objective-C.

This doesn't break anything, it just opens another door to extend the library without the need to change library classes (which makes future updates extremely unpleasant ;)).

[AlBirdie]

👍 Tanks Daniel!