Fixed file sorting to work with scrolling

The FileList.files model is now updated with file operations.
Adding files to the list will add to the model first, then to the DOM.
If the insertion point isn't visible yet, the file won't be added to the
DOM until the user scrolls down.

Updated unit tests to include checking for the correct insertion point.
Fixed insertion of files

Removed "insert" flag, inserting is by default for FileList.add().
Added "animate" flag to FileList.add().
Added logic to correctly detect when to insert/append elements whenever
the insertion point is visible or not.
Fixed "render next page" logic to work correctly when many pages of
files have been added.