persisted plugins state

Summary:
Two pros are passed into every plugin to persist state:
- `this.props.persistedState` which is the object of the persisted state
- `this.props.setPersistedState` which can be used to modify the persisted state

The state itself is stored in redux and therefore persisted when switching plugins.

The lifecycle hooks used a HOC are now implemented by the `ref`-function, which makes the code a little cleaner.

Reviewed By: jknoxville

Differential Revision: D8752097

fbshipit-source-id: d4f081f149cd840a29f1132bde91d72d3fba67ed

src/reducers/pluginStates.js

@@ -26,7 +26,10 @@ export default function reducer(
   if (action.type === 'SET_PLUGIN_STATE') {
     return {
       ...state,
-      [action.payload.pluginKey]: action.payload.state,
+      [action.payload.pluginKey]: {
+        ...state[action.payload.pluginKey],
+        ...action.payload.state,
+      },
     };
   } else {
     return state;