Shift click now available to select file ranges

New vars added for the new event of shift-clicking to select ranges of
files in the file manager
Get the last file clicked and work out if the file we've just clicked is
alphabetically greater than it or not, so we end up with a startFile <
endFile and are therefore selecting from startFile downwards
In a for loop, run until we find our startFile and begin selecting.
If we're selecting and the file we're looking at isn't in our
selectedFiles array, select that file and add it to the array
Finally, when we reach our endFile, break out of the loop