Update Ghoul version

Update Milkyway textures to use png instead of pbm
Update Exoplanets to use png instead of pbm

Closes #707 

Disable SPOUT module on non-Windows machines

 * Move global objects out of OpenSpaceEngine
 * General cleanup of main.cpp
 * Add default_joystick asset to all scenes
 * No longer suppress mouse interaction on slave nodes
 * Window delegate uses function pointers rather than subclassing
 * Fix for false overwriting of ImGUI configuration file
 * Change default color and tilt angle of fisheye rendering
 * Restructured performance manager
 * Simplify screenshot handling

Adapt to glbindings changes

Remove extra opacity value from renderablebillboardscloud and make text labels listen to the opacity (closes #693)

Issue/675