Implement showing window icons in titlebar (#4439)

This feature defaults to off, and can be turned on for individual windows,
or (with for_window) for all new windows. See the userguide change.

This commit is partially based on work by:

• Marius Muja
• mickael9
• Esteve Varela Colominas
• Bernardo Menicagli

src/handlers.c

@@ -1185,6 +1185,14 @@ static bool handle_i3_floating(Con *con, xcb_get_property_reply_t *prop) {
     return true;
 }
 
+static bool handle_windowicon_change(Con *con, xcb_get_property_reply_t *prop) {
+    window_update_icon(con->window, prop);
+
+    x_push_changes(croot);
+
+    return true;
+}
+
 /* Returns false if the event could not be processed (e.g. the window could not
  * be found), true otherwise */
 typedef bool (*cb_property_handler_t)(Con *con, xcb_get_property_reply_t *property);
@@ -1208,7 +1216,8 @@ static struct property_handler_t property_handlers[] = {
     {0, UINT_MAX, handle_window_type},
     {0, UINT_MAX, handle_i3_floating},
     {0, 128, handle_machine_change},
-    {0, 5 * sizeof(uint64_t), handle_motif_hints_change}};
+    {0, 5 * sizeof(uint64_t), handle_motif_hints_change},
+    {0, UINT_MAX, handle_windowicon_change}};
 #define NUM_HANDLERS (sizeof(property_handlers) / sizeof(struct property_handler_t))
 
 /*
@@ -1232,6 +1241,7 @@ void property_handlers_init(void) {
     property_handlers[10].atom = A_I3_FLOATING_WINDOW;
     property_handlers[11].atom = XCB_ATOM_WM_CLIENT_MACHINE;
     property_handlers[12].atom = A__MOTIF_WM_HINTS;
+    property_handlers[13].atom = A__NET_WM_ICON;
 }
 
 static void property_notify(uint8_t state, xcb_window_t window, xcb_atom_t atom) {