Address stability problems for camera paths and make them more useful in general

* Change spline curve parameter interval from [0,1] to [0,nSegments], for slightly increased precision
* Use linear paths whenever precision isn't enough/paths are too long (and traverse linear paths differently, to make them less depending on the distance)
* Remove OrbitalNavigator.LinearFlight and instead add "Zoom to" helper functions (closes #1837 )
* Add check to prevent triggering awkward short paths, for example when flying to the current target or to the same geo position twice
* Refactor speed code and handle speed when path is shorter than start+closeup distance
* Add easing to speed dampening
* Add topic for engine mode (to be used for UI later)
* Cleaner log when creating paths (some previous info messages are now debug messages)

Related PR: OpenSpace/OpenSpace-WebGuiFrontend#70

src/engine/openspaceengine.cpp

@@ -1180,6 +1180,17 @@ void OpenSpaceEngine::preSynchronization() {
         setCameraFromProfile(*global::profile);
         setAdditionalScriptsFromProfile(*global::profile);
     }
+
+    // Handle callback(s) for change in engine mode
+    if (_modeLastFrame != _currentMode) {
+        using K = CallbackHandle;
+        using V = ModeChangeCallback;
+        for (const std::pair<K, V>& it : _modeChangeCallbacks) {
+            it.second();
+        }
+    }
+    _modeLastFrame = _currentMode;
+
     LTRACE("OpenSpaceEngine::preSynchronization(end)");
 }
 
@@ -1617,6 +1628,31 @@ void OpenSpaceEngine::resetMode() {
     LDEBUG(fmt::format("Reset engine mode to {}", stringify(_currentMode)));
 }
 
+OpenSpaceEngine::CallbackHandle OpenSpaceEngine::addModeChangeCallback(
+                                                                   ModeChangeCallback cb)
+{
+    CallbackHandle handle = _nextCallbackHandle++;
+    _modeChangeCallbacks.emplace_back(handle, std::move(cb));
+    return handle;
+}
+
+void OpenSpaceEngine::removeModeChangeCallback(CallbackHandle handle) {
+    const auto it = std::find_if(
+        _modeChangeCallbacks.begin(),
+        _modeChangeCallbacks.end(),
+        [handle](const std::pair<CallbackHandle, ModeChangeCallback>& cb) {
+            return cb.first == handle;
+        }
+    );
+
+    ghoul_assert(
+        it != _modeChangeCallbacks.end(),
+        "handle must be a valid callback handle"
+    );
+
+    _modeChangeCallbacks.erase(it);
+}
+
 void setCameraFromProfile(const Profile& p) {
     if (!p.camera.has_value()) {
         throw ghoul::RuntimeError("No 'camera' entry exists in the startup profile");