Cleanup source tree

Move all C++ source files in the src subfolder. Adapted the cmake process accordingly and cleanup.

src/Overlay.cpp

@@ -0,0 +1,23 @@
+#include "Overlay.h"
+
+Overlay::Overlay()
+{
+
+}
+
+SnapshotOverlay::SnapshotOverlay() : Overlay()
+{
+
+}
+
+void SnapshotOverlay::draw()
+{
+
+}
+
+
+void SnapshotOverlay::update (float dt)
+{
+
+
+}