tree/container: introduce container_is_sticky[_or_child] functions

To query whether a container is sticky, checking `con->is_sticky` is
insufficient. `container_is_floating_or_child` must also return true;
this led to a lot of repetition.

This commit introduces `container_is_sticky[_or_child]` functions, and
switches all stickiness checks to use them. (Including ones where the
container is already known to be floating, for consistency.)

sway/tree/container.c

@@ -1609,3 +1609,11 @@ bool container_is_scratchpad_hidden_or_child(struct sway_container *con) {
 	con = container_toplevel_ancestor(con);
 	return con->scratchpad && !con->workspace;
 }
+
+bool container_is_sticky(struct sway_container *con) {
+	return con->is_sticky && container_is_floating(con);
+}
+
+bool container_is_sticky_or_child(struct sway_container *con) {
+	return container_is_sticky(container_toplevel_ancestor(con));
+}