Introduce engine modes to handle Camera path and session recording transitions

* Clarifies which system is control over time and camera at what time
* Fixes #1845
* Prevent invalid switching between session recoring and camera path playback
* Some cleanup, mosly of SessionRecording and NavigationHandler. Also, remove ExternInteraction and some other unused/nonexisting functions

src/camera/camera.cpp

@@ -24,6 +24,7 @@
 
 #include <openspace/camera/camera.h>
 
+#include <openspace/camera/camerapose.h>
 #include <sstream>
 
 namespace openspace {
@@ -43,6 +44,11 @@ Camera::Camera(const Camera& o)
     , _cachedLookupVector(o._cachedLookupVector)
 {}
 
+void Camera::setPose(CameraPose pose) {
+    setPositionVec3(std::move(pose.position));
+    setRotation(std::move(pose.rotation));
+}
+
 void Camera::setPositionVec3(glm::dvec3 pos) {
     if (!glm::any(glm::isnan(pos))) {
         std::lock_guard _lock(_mutex);