Show loading widget at bottom of loading lists

[DouglasValerio]

What is the content type?

• Improvement

Why is this change necessary?

• A list of widgets with some kind of 'infinite scroll + load as you go' is a common pattern we encounter when developing mobile apps. Usually, the user expects some sort of indication that more information is loading at the bottom of said list.

How does this address the issue?

• The package already provided a well-thought implementation of a loading widget, which was shown when the resource state was set to loading, so we took advantage of that and only treated the case for when the Resource is in the loading state, but already has some data from previous data-fetch calls.

• Firstly, we made sure that if the resource has some data, the length of the rendered list takes that into account, the actual implementation is quite simple, and can be viewed in: e1f5820

• On the second pass, we made some modifications on the _generateListBuilder(...Args) method, so it can keep rendering the existent data while also showing the loading widget at the bottom.

• The main change to the aforementioned method was the introduction of the _handleLoadingState(..Args) which takes upon the role of performing some checks on whether the _generatedListBuilder should return a tile with rendered data or the loadingWidget itself.

• The most sensitive task performed by the _handleLoadingState method is to ensure that we don't run into a range exception, once now, in the loading state we are rendering a list whose length is greater than the one on the Resource itself.

• The implementation of this method can be checked at: a4d7204

[LucazzP]

In that case, will not be better to return this _handleLoadingState if is with Status.loading or Status.success?
Because you added the success logic on this function, so the correct name for this function should be _mapIndexToTile, or something like that... What you think?