Introduce support for categorizing plugins

Summary:
This PR introduces the possibility to group plugins in categories.
The category can be determined by setting the `category` field in `package.json`.
Categories are sorted alphabetically.
Categories are shown below all uncategorized items.
Within categories, items are sorted as before: by last recently usage.

Design wise, the category name might now look more prominent than the app name, this is to be addressed in a follow up PR.

Reviewed By: jknoxville

Differential Revision: D18169459

fbshipit-source-id: 77deb0f27a0462a0d449944ddc262396160687a2

src/plugins/seamammals/package.json

@@ -1,12 +1,13 @@
 {
-    "name": "sea-mammals",
-    "version": "1.0.0",
-    "main": "index.tsx",
-    "license": "MIT",
+  "name": "sea-mammals",
+  "version": "1.0.0",
+  "main": "index.tsx",
+  "license": "MIT",
   "keywords": ["flipper-plugin"],
-    "icon": "apps",
-    "title": "Sea Mammals",
-    "bugs": {
-      "email": "realpassy@fb.com"
-    }
+  "icon": "apps",
+  "title": "Sea Mammals",
+  "category": "Example Plugin",
+  "bugs": {
+    "email": "realpassy@fb.com"
   }
+}