Fix last two bindings in input_mapping demo

Also connect the signal from script, as it makes it easier to understand
than by having to check the connected signals via the GUI.

(cherry picked from commit 4eb49cc732)

demos/gui/input_mapping/controls.gd

@@ -51,4 +51,6 @@ func _ready():
 		# We assume that the key binding that we want is the first one (0), if there are several
 		input_event = InputMap.get_action_list(action)[0]
 		# See note at the beginning of the script
-		get_node("bindings").get_node(action).get_node("Button").set_text(OS.get_scancode_string(input_event.scancode))
+		var button = get_node("bindings").get_node(action).get_node("Button")
+		button.set_text(OS.get_scancode_string(input_event.scancode))
+		button.connect("pressed", self, "wait_for_input", [action])