Closing the files pane does not collapse it

[dimitrovs]

If we close the files pane it disappears but leaves an empty div which takes up valuable real estate. When the files pane is closed we should set the width of its grid-section class to 0% and adjust the left of the next grid-section to 0, with width: 100%. This will allow the editor to expand and take up all available real estate. Re-opening the file pane should undo the width changes.