Feature #4: Default active window opacity

- Add default active window opacity (--active-opacity). (#4)

- Add win_focusin and win_focusout D-Bus signals.

src/common.h

@@ -424,6 +424,8 @@ typedef struct {
   /// 32-bit integer with the format of _NET_WM_OPACITY. 0 stands for
   /// not enabled, default.
   opacity_t inactive_opacity;
+  /// Default opacity for inactive windows.
+  opacity_t active_opacity;
   /// Whether inactive_opacity overrides the opacity set by window
   /// attributes.
   bool inactive_opacity_override;
@@ -1711,6 +1713,12 @@ cdbus_ev_win_mapped(session_t *ps, win *w);
 
 void
 cdbus_ev_win_unmapped(session_t *ps, win *w);
+
+void
+cdbus_ev_win_focusout(session_t *ps, win *w);
+
+void
+cdbus_ev_win_focusin(session_t *ps, win *w);
 //!@}
 
 /** @name DBus hooks