Change Explanations to Comments

## Feature
### User Story
As an user i want to be able to add multiple comments to an Act/Fact/Duty/Flintsource so i can explain certain parts of the model to other users. A single explanation field is not sufficient for this goal.
### Description
Currently the explanation field is a single string property which can be edited. Since the user wants to be able to add multiple explanations and call them 'Comments' this feature needs to be changed. The changes consist of removing the string property and instead of this adding a child collection of 'Comment'. 'Comment' itself has a single string property which can be edited to contain a comment. An additional benefit having 'Comment' as an child is future expansion. Additional features can be added to child collections which are not possible on string properties. Explanation intentions/constraints/generation/importing/exporting need to be altered to support this change.