Remove ARGB as a window mode

Instead use win_has_alpha to check for alpha channel, window mode is
reserved for determine if the window is possibly transparent.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/win.h

@@ -95,3 +95,6 @@ static inline Window
 win_get_leader(session_t *ps, win *w) {
   return win_get_leader_raw(ps, w, 0);
 }
+
+/// check if window has ARGB visual
+bool win_has_alpha(win *w);