Add sdl mixer

You can now play audio and sound effects! I can guarantee that this doesn't work on Windows or Mac, but this is a start. I only got it working on my Linux.

Need to get the lib-files (.dll and .a) for Windows and do some more testing so this actually works on all platforms.

dependencies/subprojects/animationwindow/include/AnimationWindow.h

@@ -10,6 +10,7 @@
 #include "Color.h"
 #include "Font.h"
 #include "Image.h"
+#include "Audio.h"
 #include "KeyboardKey.h"
 #include "Point.h"
 #include "Widget.h"
@@ -129,5 +130,8 @@ class AnimationWindow {
     void setBackgroundColor(TDT4102::Color newBackgroundColor);
 
     float get_delta_mouse_wheel() const;
+
+    // 
+    void play_audio(TDT4102::Audio& audio, int loops = 0);
 };
 }  // namespace TDT4102