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:

• [x] Use store for dropdown state #47
• [x] Use store for search #63
• [ ] Use Svelte store for all bookmarks #48
• [ ] Use derived store for directories #49