Simplify useWindowStore

[bonbud-macryg]

useWindowStore is too complicated and too opinionated; this is causing problems with rendering, and it makes state updates that should be simple more complicated than they need to be.

The main bit of frontend state — windowTree — should be changed to a map of number to string | null. The numbers will represent window IDs, and the strings will be the paths that these windows have open.

I like the way the window manager behaves, where each node has two children, and would like to keep that binary-tree structure. This structure should be implied in the numbering in the map: each number is either n*2 or (n*2)+1 of a previous number in the map, and from that the windowContainer can build a tree of windows and window containers.

A nice-to-have for this PR is fixing the UI bug where splitting a window causes an unsightly flash where the space is empty for a few milliseconds. I think this can be fixed like so:

• Whatever else, avoid a situation where "splitting a window" means unmounting and remounting its parent in the DOM.
• The rendering logic, which will live in WindowContainer, is simple. For every entry in the map, we have props id and path. If that entry is a node which could have a left and right child, we can infer that the ids of those children will be id*2 and (id*2)+1 respectively. If the path prop is null, we know that this window is not rendering content, but more windows. We don’t want to update the path of this node if we add a child to it, so we just say this: if a node’s path is null, render the window whose ID is id*2 as the left child of this node.
• The default value of the app state is a map with two K-V pairs: [1, null] and [2, ‘~sampel/home’].
• The above displays one “window” as far as the user is concerned. To “split this window” is to just add a third K-V pair to the map: [3, ‘~sampel/path’], where “~sampel/path” is just a placeholder for whatever the real path is.
• React will notice this state change and then update the UI accordingly; given the above, it should be possible to do this without visibly unmounting and remounting the existing window at 2 in the map, but I’m not sure exactly what the JSX will look like.
• I suspect you want to render two Allotments per node, even if the node only has one child. To “split a window” is then to just add an Allotment.Pane to the empty Allotment next to “the window” that we’re seeing.
• This will all still involve a recursive WindowContainer component and paying attention to the key attribute; maybe the WindowContainer should try to render the n*2 child beneath it even if no such K-V pair exists in the map, and then the WindowContainer trying to render that child needs to render the empty Allotment with that non-existent key so that when a new node is added, React knows that the newly-created Allotment with key 4 is replacing the already-existing Allotment with key 4.