Extended the code to handle multiple result objects.

For CAS like Maxima the user input can contain multiple commands where separate results are calculated and retrieved, e.g. something like 2+2;5+5;.
Befor this change Cantor would add the two results to one single result graphic object which is wrong. Internally, maxima handles these results totally independently.
Every result has it's own %o label and we should treat this appropriately.

Maxima code was adjusted to handle multiple results. In future we should also add/show the different output labels %o to the graphic objects so it is clear to the user how to reference them.
Also, we need to check other systems like Sage, Octave etc. for whether they also produce multiple results for such multi-command entries.