Rename project to trailhead-path-bookmarker

- npm name: trailhead-path-bookmarker (package.json + lockfile)
- electron-builder appId: com.dbeuttel.trailhead-path-bookmarker
- AppUserModelId in main.js matches the new appId
- README title updated; userData path now %APPDATA%\Trailhead Path Bookmarker

Icon generator now emits both sizes:
- assets/tray-icon.png (32x32) for the system tray
- assets/app-icon.png (256x256) for electron-builder's win.icon, which
  rejects PNGs smaller than 256

package-lock.json

@@ -1,11 +1,11 @@
 {
-  "name": "folder-bookmark-tray",
+  "name": "trailhead-path-bookmarker",
   "version": "0.1.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
-      "name": "folder-bookmark-tray",
+      "name": "trailhead-path-bookmarker",
       "version": "0.1.0",
       "hasInstallScript": true,
       "license": "MIT",