Interdependent items builder

[rafaelsetragni]

Due to difficulties implementing a chat app using the GroupedListView widget with the current itemBuilder and indexedItemBuilder methods, I added a new builder called interdependentItemBuilder to construct the items while bringing in the previous, current, and next elements on each step, resulting in better performance.

I also added:

• A complete test case to verify that the previous, current, and next elements are delivered in the expected order, in various list sizes and sort orders using the new interdependentItemBuilder method.
• Support for the Android and iOS platforms in the example app, allowing me to test the app's performance and usability on real devices.
• An improved chat example app to fix layout mistakes in the previous example, and test its ability to expand the keyboard over the chat list, rebuild the chat list on each infinite scroll reload, and maintain good performance during testing.

Here is a print screen of the new Chat Example App:

[Dimibe]

Thank you for your contribution, lgtm!