core: Make use of [[nodiscard]] with the System class

Given this is a central class, we should flag cases where the return
value of some functions not being used is likely a bug.

src/core/core.cpp

@@ -632,7 +632,11 @@ const std::string& System::GetStatusDetails() const {
     return impl->status_details;
 }
 
-Loader::AppLoader& System::GetAppLoader() const {
+Loader::AppLoader& System::GetAppLoader() {
+    return *impl->app_loader;
+}
+
+const Loader::AppLoader& System::GetAppLoader() const {
     return *impl->app_loader;
 }