Replace callbacks in activities/fragments with Fragment Result API

[AndreThlckr]

Currently, in order to retrieve the value from the CalcDialog, the owner (an activity or a fragment) needs to implement the CalcDialog.CalcDialogCallback interface.

In the newer versions of the fragment library, the Fragment Result API is available, which I believe is a better way to implement the communication between the dialog and the owner. (Fragment Result API)

It's usage would be something like this:

setFragmentResultListener("calcDialogKey") { requestKey, bundle ->
	val enteredValue = bundle.getSerializableExtra("enteredValueKey") as BigDecimal
	// Handle the result...
}

Some utility functions could also be created to simplify even more this process, resulting in something like:

setCalculatorResultListener { enteredValue: BigDecimal ->
	// Handle the result...
}

If this seems like a good idea, I can work on this feature and open a PR with the changes required.

[maltaisn]

Sure, it seems like that's the way to go going forward. You can work on a pull request and I'll merge it.