Xmgr: double clicks

## Double clicks

When activated, double clicking on parts of the canvas will invoke certain popups.

left of graph frame
  y axis label/ticks
below graph frame
  x axis label/ticks
on focus markers
  move selected viewport corner
within graph frame
  Edit nearest set symbols or Read sets if none active
right of graph frame
  Legend
above graph frame
  Titles/subtitles

This behavior can be turned on or off in the Options/Misc menu.