Use Svelte stores for state management

[peterhil]

Notes:

• Make components represent the state in a simple view and not have much logic or state on their own.
• Remember that Svelte store contract is implemented by reactive streams (RXJS/Kefir observervables), but do not overuse Kefir when simpler solutions would work.

Tasks:

• Use store for dropdown state Fix dropdown logic to use a store #47
• Use store for search Make CategorySelector code elegant #63
• Use Svelte store for all bookmarks Use store for all bookmarks #48
• Use derived store for directories Use derived store for all directories #49