Add About page for Khoj to Desktop app. Expose it via system tray

- Pass current khoj version from package.json to about page via
  electron IPC between backend js and frontend page
- Update Khoj information in default About screen as well, in case
  it's exposed anywhere else

src/interface/desktop/preload.js

@@ -52,3 +52,7 @@ contextBridge.exposeInMainWorld('tokenAPI', {
     setToken: (token) => ipcRenderer.invoke('setToken', token),
     getToken: () => ipcRenderer.invoke('getToken')
 })
+
+contextBridge.exposeInMainWorld('appInfoAPI', {
+    getInfo: (callback) => ipcRenderer.on('appInfo', callback)
+})