Restructure cofolded molecules

[j-4]

Cofolded molecules should have one RNAgraph representation per molecule, and not one RNAgraph for both molecules.

To achieve this, we would need to call the outlaying algorithm with both molecules concatenated, extract the positions for the two molecules individually and also extract the intra-moleculare basepairs to be included as extra links.

This approach will fix the labeling problem (#9) and the external loop problem (#10).
Moreover, it is more universal when it comes to coloring and deleting individual molecules.
It will also get rid of a lot of bloated code in the fornac class.

[j-4]

once an API is set, it can be integrated into forna ViennaRNA/forna#34

[j-4]

this will also fix #9