Use proper locking instead of splash screen lock

Partial fix for splash screen deadlocks. 

* Will partially break splash screen.
   * Splash screen remains white for a long time

 * Old code syncronized LaunchFrame thread with
   "loader.setVisible(true)"
   * Splash screen controls main thread
   * downloadServersReady() didn't check if splashsceen
     was visible which caused deadlocks

 * TODO: find and convert remaining similar locking codes

Signed-off-by: Janne Kujanpää <jikuja@iki.fi>