[CenterOfMass] Added support to enter a mass explicitly

[nexx512]

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[nexx512]

This pull request is a follow up to #130 due to a changed source branch with corrections mentioned in https://github.com/FreeCAD/FreeCAD-macros/pull/130#issuecomment-1113411721

[s-quirin]

Thank you for your contribution! I have a few comments and inconsistencies:

• [ ] You could already merge your part with #129
• [ ] Save (as) was a compromise with @chupins. "Transfer" is also okay, I even more like "to Doc.". You could also think about the icon then, e.g. Std_MergeProjects.svg
• [ ] Material density has a physical range, mass has not. How do you want to adjust the decimals in your spin box accordingly? The scientific notation is probably the better one to cover multiple order of magnitude.
• [ ] Your implementation affects Import and Export. You could think about having a Material "fixed mass" instead of checkbox.
• [ ] The density should change accordingly if you change mass, or the colouring should be disabled for this solid.

[nexx512]

• Having a "fixed mass" in the dropdown seems to be a reasonable alternative to a checkbox. I can check if I can do that.
• I didn't notice that the spinbox was limited to 99.99... There needs to be a better option. Maybe I check for reasonable ranges.
• I didn't look into the import and export feature...totally missed that.

[nexx512]

ok, I tried to refactor my changes but with no luck. The design of this macro is way too complicated to be able to see what is happening. For example the spinDens is taken out of the SolidWidget object and manipulated in many other places. It's very hard to keep track of the paces in the code that need to be considered when a change is made. It's only 1000 lines of code but I wasn't able to find a clear structure, so I gave up. Sorry, but i guess this macro should be refactored with a clear and understandable architecture.