Feature/minvr (#744)

* Move SGCT external library from ext/sgct to apps/OpenSpace/ext/sgct
 * Remove all GLFW dependencies from openspace-core
 * Add MinVR dependency
 * Add new executable OpenSpace-MinVR that uses MinVR instead of SGCT

include/openspace/engine/openspaceengine.h

@@ -79,8 +79,9 @@ public:
     void mouseButtonCallback(MouseButton button, MouseAction action);
     void mousePositionCallback(double x, double y);
     void mouseScrollWheelCallback(double posX, double posY);
-    void encode();
-    void decode();
+    void externalControlCallback(const char* receivedChars, int size, int clientId);
+    std::vector<char> encode();
+    void decode(std::vector<char> data);
 
     void scheduleLoadSingleAsset(std::string assetPath);
     void toggleShutdownMode();