various small code cleanups and improvements

Largely refactoring work to make the code somewhat nicer to work with.

app/main.cpp

@@ -85,13 +85,13 @@ int main(int argc, char *argv[]) {
 #endif
 
   // Start core
-  olive::core.Start();
+  Core::instance()->Start();
 
   // Run application loop and receive exit code
   int exit_code = a.exec();
 
   // Clear core memory
-  olive::core.Stop();
+  Core::instance()->Stop();
 
   return exit_code;
 }