Update "Make Available Offline" code for nautilus API break
The "Shortcuts" window lists "Ctrl+S" for "Make Available Offline". That either does not seem to work in 3.30 (I pressed that while watching a remote video and then went offline to check), or if it works under some circumstances then it is non-obvious.
If this functionality is not supported, its shortcut should be removed.
Also see #193 (closed) and #191 (closed).
Edited by Bastien Nocera