Added FileList.setViewerMode to hide controls

Some files app embed themselves under the controls (like the text
editor). The new method FileList.setViewerMode() makes it possible to
properly show/hide the control buttons using the correct permissions.

Apps using this approach must call setViewerMode(true) when starting and
setViewerMode(false) upon closing to restore the controls.

This is needed for #5284

Please review @karlitschek @schiesbn @jancborchardt @Kondou-ger