Move editor functions to separate header

Editor functions now live in Editor.h, which is a new file - the
existing editor.h has been renamed to CustomLevels.h.

desktop_version/src/Editor.h

@@ -0,0 +1,16 @@
+#if !defined(NO_CUSTOM_LEVELS) && !defined(NO_EDITOR)
+
+#ifndef EDITOR_H
+#define EDITOR_H
+
+void editorrender(void);
+
+void editorrenderfixed(void);
+
+void editorlogic(void);
+
+void editorinput(void);
+
+#endif /* EDITOR_H */
+
+#endif /* NO_CUSTOM_LEVELS and NO_EDITOR */