Add generic type constraint

[michalsrutek]

Basically added type constraints to

final List<T> Function(String) filter;

and

class _SearchableListState<T> extends State<SearchableList<T>> {

Which makes it easier to work with, as you get a correct type in filter function and gets rid of annoying errors such as

'(Actor) => Padding' is not a subtype of type '(dynamic) => Widget'

[koukibadr]

Surely that's a good enhancement indeed,
thanks @michalsrutek for contributing I will publish it ASAP to pub.dev