Animated Splash screen

This PR shows an animated splash screen during initialization phase. It is platform-independent (in contrast to launch4j) and also shows progress messages.

Unfortunately, LWJGL 2 does not allow for creating a hidden display window. As a consequence, the window still shows black for 1-2 seconds while shaders, etc. are loading. In LWJGL 3, window visibility can be toggled though.

Closes #1454