Add Import option to the Models Dialog

[leozide]

Discussed in https://github.com/leozide/leocad/discussions/898

^{Originally posted by MrEdgeNL January 7, 2024} Hello,

Would it be possible having an "import" function, next to the existing "export" function in the "SubModels..." menu?

I understand, when importing a LDR file, which also includes submodels, it starts to be fuzzy. So in this case, importing everything till the first "NoFile" command would work?

Ofc. it's also possible to copy-paste a model/submodel from a second LeoCAD-instance, but this will NOT copy the STEP information.

Best regards, Koos.

(Simular to this question: https://github.com/leozide/leocad/discussions/779)