Now we restore the comparator that was used before pause a FileListFragment
First approach to double-click events within a ListView. It uses a GestureDetector to detect double-click events.
Fixed a bug in DoubleClickSupportedListView.

Added double-click support to FileListFragment. To do so, It takes advantage of DoubleClickSupportedListView.

In addition, a custom view is added to the fragment. This let us include the empty list text, however we have lost the ability to automatically show the ProgressBar when the Adapter is not set. To solve that we will include our so known ListFragmentCustomLayout class.
Improve doc
FileListFragment modified to inherit from ListFragmentCustomLayout. Now we can show again the progress bar when DropboxManager is fetching files.

In addition, ListFragmentCustomLayout includes a foot element with a ProgressBar, so we can use it in an endless list style.
Added a method to DropboxManager that partially explores the directory tree.

Also, I prepare FileListFragment for partial exploration although it still uses the DropboxManager.getAllFiles method.
EndlessSortedListAdapter added. This class automatically resquest more data when the user scrolls down.
Fixed some bugs in EndlessSortedListAdapter. Now it starts automatically to fetch data. Also, it stops automatically when we add a collection, however if we add a single element we must call setNoMoreDataToLoad()
FileListFragment modified to use partial files listing. FileListFragment also stores the list of directories to inspect in future requests.

For sure this will make a performance difference when dealing with Dropbox accounts with hundreds of files. I configured it to download 20 elements on each request.
Added a custom adapter (and custom view) for Dropbox files.
Fixed a bug. When coming back from BookDetailsFragment the adapter was created again but there wasn't paths to explore. Now we do not recreate the adapter.
list item logo was set statically in the xml. There is no reason to do it programmatically.