0.5.4: Shape fitting and mesh importer utils

0.5.4 adds support for basic shape fitting into the model editor, some nice-to-have orientation/translation actions in the mesh importer, and some bugfixes.

• Stations are now selectable when using the Translate > To (select something) and Reorient > $axis > To (select something) actions in the mesh importer (thanks @emmccain-uva, #796)
• The mesh importer now has an option to Reorient > $axis > Along line between (select two elements), which is sometimes useful (esp. in conjunction with importing stations, #149) for defining joint/body frames (thanks @emmccain-uva, #797)
• Right-clicking a mesh in the model editor now shows an Export menu, enabling re-exporting of the mesh w.r.t. a different coordinate system (#799)
• Right-clicking a mesh in the model editor now shows a Fit Analytic Geometry to This option, which will fit the chosen analytic geometry to the mesh's vertices (#798).
• If a popup in the editor window throws an exception then the editor window will now try to close the popup, rather than fully erroring out to an error tab (thanks @AdrianHendrik, #800)
• Fixed a bug where trying to add a ConditionalPathPoint, Marker, PathPoint, PhysicalOffsetFrame, or Station component via the Add menu would crash the tab with a frame-related error message (thanks @AdrianHendrik, #800)
• Internal: the codebase now contains shape-fitting algorithms for fitting a sphere, plane, or ellipsoid analytic geometry to mesh data. The algorithms were written to closely match the shape-fitting codebase that came with "P.Bishop et. al, How to Build a Dinosaur, doi:10.1017/pab.2020.46" (#798)