Adds a persistent file used to store user settings (#2931)

* Enable the loading of settings
* Add a user interface for changing the settings in the launcher


Co-authored-by: Emma Broman <emma.broman@liu.se>

tests/test_sgctedit.cpp

@@ -34,7 +34,7 @@
 #include <filesystem>
 #include <fstream>
 
-using namespace openspace::configuration;
+using namespace openspace;
 
 namespace {
     std::string stringify(const std::string filename) {