Clean up editorclass externs into one location

Again, like the previous commit, it should just be put in the header
file of its respective class instead of being a mess everywhere.

desktop_version/src/editor.h

@@ -248,6 +248,8 @@ void editorlogic();
 
 void editorinput();
 
+extern editorclass ed;
+
 #endif /* EDITOR_H */
 
 #endif /* NO_CUSTOM_LEVELS */