Improve compiler compatibility (#743)

* fix returns

* use standard C headers

* struct/class mismatch

* explicit this in template

* switch variable scope

* C standard compliance

* & l-value

include/JSystem/JStudio/JStudio/jstudio-control.h

@@ -8,7 +8,7 @@
 #include "dolphin/mtx/mtxvec.h"
 
 namespace JStudio {
-struct TObject;
+class TObject;
 struct TCreateObject {
     TCreateObject() {}
     virtual ~TCreateObject() = 0;

include/JSystem/JStudio/JStudio/stb.h

@@ -7,7 +7,7 @@
 #include "global.h"
 
 namespace JStudio {
-struct TObject;
+class TObject;
 namespace stb {
 
 class TControl;

include/JSystem/JStudio/JStudio_JAudio/control.h

@@ -5,8 +5,8 @@
 #include "JSystem/JStudio/JStudio/jstudio-object.h"
 #include "JSystem/JStage/JSGSystem.h"
 
-struct JAIBasic;
-struct JAISound;
+class JAIBasic;
+class JAISound;
 namespace JStudio_JAudio {
 typedef JStudio::TObject* (*JStudioAudioCreateFunc)(const JStudio::stb::data::TParse_TBlock_object&, JAIBasic*);
 typedef void (JAISound::*TVVOSoundSetFunc)(f32, u32); // e.g. JAISound::setDirectVolume, JAISound::setDirectPitch