Rearrange structure of the project

Previously:

- api
...
- Components
  - Canvas
  - edition
  - Execution
  - Frontpage
  - General
  ...
- CustomEdges
- CustomNodes
...

The structure had several drawbacks:

• The Components folder is misnamed since it also contains utils and types
• Some components (CustomEdges and CustomNodes) were out of the Components folder
• There is a hierarchy of components due to introduction of edition and execution pages which was not respected.

Now:

- api
...
- edition
  - Canvas
  - CustomEdges
  - CustomNodes
  ...
- execution
- frontpage
- general
...

• Edition components, models and types are in the edition folder. Same for execution.
• Components not related to a particular page (like frontpage, general and suspense) are in folders on the same level as edition and execution. Although it remains to be checked that some edition components are not in general.
• Top-level folder names are now all lowercase