NEW FEATURE - Measure star displacement in focus / align / framing

[Paul Shears]

Add a new feature to aid set-up of mounts and to check mount tracking.

For a given star in the focus reticule, implement a mode whereby the displacement (in both X and Y) is displayed for that star from a set point (button press) to the current exposure. This can be used to aid setup of the mount with respect to tracking and also drift align EQ mounts.

Perhaps a visual 'snail trail' can be displayed showing the drift of the target star.