Removing coupling on tasks slice in store setters

For #185 (closed)

To remove tasks from the store (and use a fetch store instead), we have to deal that some setters in the store (namely setWorkingGraph and setSubgraph) are using the tasks in the store (via get().tasks) to retrieve the list of tasks. There are apparently used to deduce the inputs/outputs of the graph nodes and edges.

Before even thinking about removing, I think this way of accessing tasks is flawed. Indeed, tasks are fetched and set when mounting EditPage. For get().tasks to return an up-to-date list of tasks, we are relying on the fact that EditPage is mounted before calling get().tasks. This is indeed the case but it is an rather indirect coupling.

To solve this, I added tasks as a parameter to pass to the setters (setWorkingGraph and setSubgraph). It is up to the components calling the setters to provide an up-to-date list of tasks.

For now, it is via the existing store slice but later (!335 (merged)) it will be via the fetch store.