Allow custom definition of head groups and fragments

[nilshoffmann]

In order to allow for an easier customization of LipidCreator, an additional "Custom" tab could be added that allows the user to define new lipid classes with a defined headgroup (sum formula) + FA / LCB and corresponding fragment templates.

This would make it easier for users to also provide new lipid classes and fragments for proper inclusion and attribution in LipidCreator. Potentially, allowing to add these as SMILES would make it easier to use as well.

[chemice]

Adding headgroup and define fragments are not the problem. But the structure images will be totally missing. Maybe it is better to open a new tab for the calculation of user defined lipid classes.