Bug fix #99: Rewrite focus detection logic

- Rewrite focus detection logic. Remove w->focused_real and use
  ps->active_win to identify focused window uniformly. Use a more
  expensive way to filter FocusIn/Out events to improve reliability.
  Only limited tests are done, and bugs are likely to be introduced.
  (#99)

- Known issue: Under fvwm, compton sometimes does not consistently
  report the window input gets sent to. But there's something wrong in
  that case: XGetInputFocus() shows the root window is focused but
  another window is receiving input.

src/dbus.c

@@ -722,7 +722,11 @@ cdbus_process_win_get(session_t *ps, DBusMessage *msg) {
   cdbus_m_win_get_do(window_type, cdbus_reply_enum);
   cdbus_m_win_get_do(wmwin, cdbus_reply_bool);
   cdbus_m_win_get_do(leader, cdbus_reply_wid);
-  cdbus_m_win_get_do(focused_real, cdbus_reply_bool);
+  // focused_real
+  if (!strcmp("focused_real", target)) {
+    cdbus_reply_bool(ps, msg, win_is_focused_real(ps, w));
+    return true;
+  }
   cdbus_m_win_get_do(fade_force, cdbus_reply_enum);
   cdbus_m_win_get_do(shadow_force, cdbus_reply_enum);
   cdbus_m_win_get_do(focused_force, cdbus_reply_enum);