Changes to SGCT and OpenSpace render engine to resolve issue #589 (#865)

* Added property for aspect ratio

* Working version with aspect ratio slider controlling the FOV, and also gets updated by a window resize.

* Created horizontal fov property / GUI control that preserves aspect ratio by adjusting vertical fov to match, and updates with window resizing.

include/openspace/engine/windowdelegate.h

@@ -113,6 +113,10 @@ struct WindowDelegate {
 
     int (*currentWindowId)() = []() { return 0; };
 
+    double (*getHorizFieldOfView)() = []() { return 0.0; };
+
+    void (*setHorizFieldOfView)(float hFovDeg) = [](float) { };
+
     using GLProcAddress = void(*)(void);
 
     GLProcAddress (*openGLProcedureAddress)(const char*) =