Create Native Menu Bar with PyQt to open Search, Config webpages

- Run FastAPI server in a separate thread.
  - This allows starting both the server and gui in parallel

- Create System Tray for Khoj
  - Contains menu items that open search or config pages in browser

- Rearrange code to have only the code required to start Backend and
  GUI in the run() method
  - Move the backend setup code into a separate method

setup.py

@@ -39,6 +39,7 @@ setup(
         "pillow >= 9.0.1",
         "aiofiles == 0.8.0",
         "dateparser == 1.1.1",
+        "pyqt6 == 6.3.1",
     ],
     include_package_data=True,
     entry_points={"console_scripts": ["khoj = src.main:run"]},