Make "preferred fills" shareable across crosswords #42
Description
This will basically let you use "preferred fills" lists like custom wordlists. There's a limit of 50,000 for now (increased from way lower values recently, because we now do not store the list with every state-save, and because we now process the list piecemeal, without hanging the UI). But, if used judiciously, I suspect we can even go higher (100,000 words of average length 7 will require say 10 bytes per word, that's 1MB out of the 5 MB, and maybe we can even switch to indexedDB at some point).
The changes required for this transition will be:
- Decouple preflex storage from the crossword (probably use local storage keys like "-preflex-")
- Allow assigning names to preflex lists
- UI for picking from existing preflexes
- GC code needs to be modified to sweep through and find pointers from all crosswords to decide whether a preflex list can be deleted