Introduce I3_FLOATING_WINDOW

This patch introduces a proprietary atom I3_FLOATING_WINDOW which will be
set and maintained for floating windows and removed on tiling containers.

This allows users to select on this atom, e.g., in their compositor
configuration or in utility scripts (without using the IPC).

fixes #2223

src/floating.c

@@ -29,6 +29,34 @@ static Rect total_outputs_dimensions(void) {
     return outputs_dimensions;
 }
 
+/*
+ * Updates I3_FLOATING_WINDOW by either setting or removing it on the con and
+ * all its children.
+ *
+ */
+static void floating_set_hint_atom(Con *con, bool floating) {
+    if (!con_is_leaf(con)) {
+        Con *child;
+        TAILQ_FOREACH(child, &(con->nodes_head), nodes) {
+            floating_set_hint_atom(child, floating);
+        }
+    }
+
+    if (con->window == NULL) {
+        return;
+    }
+
+    if (floating) {
+        uint32_t val = 1;
+        xcb_change_property(conn, XCB_PROP_MODE_REPLACE, con->window->id,
+                            A_I3_FLOATING_WINDOW, XCB_ATOM_CARDINAL, 32, 1, &val);
+    } else {
+        xcb_delete_property(conn, con->window->id, A_I3_FLOATING_WINDOW);
+    }
+
+    xcb_flush(conn);
+}
+
 /**
  * Called when a floating window is created or resized.
  * This function resizes the window if its size is higher or lower than the
@@ -260,19 +288,19 @@ void floating_enable(Con *con, bool automatic) {
     /* Check if we need to re-assign it to a different workspace because of its
      * coordinates and exit if that was done successfully. */
     if (floating_maybe_reassign_ws(nc)) {
-        ipc_send_window_event("floating", con);
-        return;
+        goto done;
     }
 
     /* Sanitize coordinates: Check if they are on any output */
     if (get_output_containing(nc->rect.x, nc->rect.y) != NULL) {
-        ipc_send_window_event("floating", con);
-        return;
+        goto done;
     }
 
     ELOG("No output found at destination coordinates, centering floating window on current ws\n");
     floating_center(nc, ws->rect);
 
+done:
+    floating_set_hint_atom(nc, true);
     ipc_send_window_event("floating", con);
 }
 
@@ -318,6 +346,7 @@ void floating_disable(Con *con, bool automatic) {
     if (set_focus)
         con_focus(con);
 
+    floating_set_hint_atom(con, false);
     ipc_send_window_event("floating", con);
 }