showbase: allow attaching default MouseWatcher in attachInputDevice

This makes it easier to control GUIs using a gamepad.

direct/src/showbase/ShowBase.py

@@ -1677,13 +1677,17 @@ class ShowBase(DirectObject.DirectObject):
         return self.mouseWatcherNode.getModifierButtons().isDown(
             KeyboardButton.meta())
 
-    def attachInputDevice(self, device, prefix=None):
+    def attachInputDevice(self, device, prefix=None, gui=False):
         """
         This function attaches an input device to the data graph, which will
         cause the device to be polled and generate events.  If a prefix is
         given and not None, it is used to prefix events generated by this
         device, separated by a hyphen.
 
+        The gui argument can be set to True (as of Panda3D 1.10.3) to set up
+        the default MouseWatcher to receive inputs from this device, allowing
+        it to control user interfaces.
+
         If you call this, you should consider calling detachInputDevice when
         you are done with the device or when it is disconnected.
         """
@@ -1694,13 +1698,17 @@ class ShowBase(DirectObject.DirectObject):
         idn = self.dataRoot.attachNewNode(InputDeviceNode(device, device.name))
 
         # Setup the button thrower to generate events for the device.
-        bt = idn.attachNewNode(ButtonThrower(device.name))
-        if prefix is not None:
-            bt.node().setPrefix(prefix + '-')
+        if prefix is not None or not gui:
+            bt = idn.attachNewNode(ButtonThrower(device.name))
+            if prefix is not None:
+                bt.node().setPrefix(prefix + '-')
+            self.deviceButtonThrowers.append(bt)
 
         assert self.notify.debug("Attached input device {0} with prefix {1}".format(device, prefix))
         self.__inputDeviceNodes[device] = idn
-        self.deviceButtonThrowers.append(bt)
+
+        if gui:
+            idn.node().addChild(self.mouseWatcherNode)
 
     def detachInputDevice(self, device):
         """