Minor improvements to store slices

In addition to #184 (closed), I think other store slices could also be improved/simplified:

openDraggableDialog and taskDrawerState could be converted to local states !359 (merged)
openSnackbar could expose a more consumer-friendly API (like showWarning(string) or showError(string) instead of forcing consumers to set the full SnackbarParams object) !346 (merged)
tasks could be replaced by a simple fetch cache !335 (merged)
~~recentGraphs too ?~~ (No, since it is used for several things. See discussions on #196 (closed))
subGraph seems unused ? There is a setSubGraph call in AddSubgraphButton but the set subgraph does not seem to be used. !343 (merged)

Edited Sep 13, 2023 by Loic Huder

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