Added planetocentric coordinates and limb/ring distances to click location coordinates in the GUI

Added new values to the 'Coords' tab (and copyable strings etc.) when clicking on the image in the PlanetMapper GUI:

Other minor associated GUI improvements:

Added ability to have tooltips for 'Coords' tab values. Currently we only use these for the limb distance and ring radius values, as these could be a bit more ambiguous with only the short labels.
Added keyboard shortcut Control+Shift+C to formatted (i.e. human-readable) click coordinate values.
On MacOS, added keyboard shortcut aliases so that the command key can be used instead of control. For example, Command+S and Control+S both now open the save observation dialog on Mac. This only applies to MacOS (platform.system() == 'Darwin'), so Linux and Windows are unchanged and only accept Control+... shortcuts.

Closes #391

[x] Add a clear description of the change
[x] Add any new tests needed
[x] Run spell check on new text visible to user (documentation, GUI etc.)
[x] Check any changes to requirements.txt are reflected in setup.py and conda-forge feedstock
[x] Check code passes CI checks (run run_ci.sh or check GitHub Actions)

See CONTRIBUTING.md for more details.

ortk95 / planetmapper