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/c2.c

@@ -1055,7 +1055,7 @@ c2_match_once_leaf(session_t *ps, win *w, const c2_l_t *pleaf,
             case C2_L_PFULLSCREEN: tgt = win_is_fullscreen(ps, w); break;
             case C2_L_POVREDIR: tgt = w->a.override_redirect;   break;
             case C2_L_PARGB:    tgt = (WMODE_ARGB == w->mode);  break;
-            case C2_L_PFOCUSED: tgt = w->focused_real;          break;
+            case C2_L_PFOCUSED: tgt = win_is_focused_real(ps, w); break;
             case C2_L_PWMWIN:   tgt = w->wmwin;                 break;
             case C2_L_PCLIENT:  tgt = w->client_win;            break;
             case C2_L_PLEADER:  tgt = w->leader;                break;