Add config var: "expand story tree"

[Obiwarn]

and/or add a button to expand/collapse the whole tree of story folders, to get a better overview.

e.g. that it looks like this at startup:

[cm-dyoshikawa]

@tajo Could I take over his work? It seems like there hasn't been any movement for a while.

[Wesleyzxc]

@tajo didn't see any follow up on this so went ahead and created a PR for it, happy to pick up this piece