Refactor layout .show/.isShown to handle branch nodes

src/core/plugins/layout/selectors.js

@@ -1,5 +1,6 @@
 import { createSelector } from "reselect"
 import { normalizeArray } from "core/utils"
+import { fromJS } from "immutable"
 
 const state = state => state
 
@@ -9,7 +10,7 @@ export const currentFilter = state => state.get("filter")
 
 export const isShown = (state, thing, def) => {
   thing = normalizeArray(thing)
-  return Boolean(state.getIn(["shown", ...thing], def))
+  return state.get("shown", fromJS({})).get(fromJS(thing), def)
 }
 
 export const whatMode = (state, thing, def="") => {
@@ -21,4 +22,3 @@ export const showSummary = createSelector(
   state,
   state => !isShown(state, "editor")
 )
-