Keep editor in sync with component state (two-way binding)

[ovidiuch]

Functionality:

• [x] Poll loaded component and update fixture contents with snapshot
• [x] Don't update fixture contents while editor is focused
• [x] Automatically focus on editor when changing fixture (prevent overriding the original fixture right after selecting it)

Remaining concerns:

• [x] Does the component render too often? i.e. should it re-render when the state is updated on interval?
  • [x] Test with Flatris

Tests:

• refreshing fixture contents
  • [x] should register interval on mount
  • [x] should clear interval on unmount
  • on callback
  • [x] should not set state when fixture is not selected
  • with fixture selected
    • [x] should not set state when editor is focused
    • with editor blurred
    • [x] should mark user input state as valid
    • [x] should serialize preview child
    • [x] should update child snapshot state
    • [x] should stringify preview child snapshot
    • [x] should update stringified child snapshot state
• [x] should set state flag on editor focus
• [x] should unset state flag on editor blur
• [x] should focus on editor on fixture click