[UI] Update library when changing games' categories (#4829)

* [UI] Update library when changing games' categories

* Refactor: change customCategories reference only

Author: arielj

src/frontend/state/GlobalState.tsx

@@ -395,11 +395,13 @@ class GlobalState extends PureComponent<Props> {
   }
 
   addGameToCustomCategory = (category: string, appName: string) => {
-    const newCustomCategories = this.state.customCategories
-
-    if (!newCustomCategories[category]) newCustomCategories[category] = []
+    const gamesInCategory = this.state.customCategories[category] || []
+    gamesInCategory.push(appName)
 
-    newCustomCategories[category].push(appName)
+    const newCustomCategories = {
+      ...this.state.customCategories,
+      [category]: gamesInCategory
+    }
 
     this.setState({
       customCategories: newCustomCategories