Similar to 976dbf7727389d022b8d884854ab2d3f8d008b50 we need to check how to support this in other backends, too. For example in python, after loading a saved Cantor python project file,
```
import os
print(__file__)
```
should print out the path of the project file, if possible.
Progress:
[x] Python: `__file__`
[ ] Julia: ([info about vars](https://discourse.julialang.org/t/how-to-get-the-path-of-a-julia-script/1728))
- [ ]`@__FILE__`
- [x] `@__DIR__`
- [ ] `PROGRAM_FILE`
[X] Kalgebra: haven't this functionality?
[X] Maxima: haven't this functionality?
[ ] Octave: `mfilename('fullpath')` should return script full path
[X] R: Info haven't found.
[X] Sage: `__file__`
[X] Scilab: no info