The order of config.spaces used to be whatever order the spaces were created
in, and there was no way back other than deleting a space and drawing it
again.
The gesture is deliberately narrow — mouse, editors only. The same tabs are
the primary way to switch spaces in View, where touch is first class, so a
drag there would compete with the tap that switches. Recorded in the spec as
"Touch editor: not exposed".
The part that needed care is not the drag. Position in the array feeds three
things: the marker placement fallback, the swipe neighbour and a positional
`floor`. So the write that stores the new order also writes down the
placement that used to depend on it: a marker with neither an explicit space
nor an area that names one gets the space it has right now. Both changes go in
one save; splitting them would leave a window in which markers move on their
own. The positional `floor` cannot be fixed from here, so the card says so
once.
Issue: #220
User-Visible: yes