More safely read the button repeat rate during the joystick auto-repeat timer.

Grabbing &(instance of GMenu2X).confInt["buttonRepeatRate"] is unsafe, because
the storage for the slot may move as the slot is deleted or added. Now a
callback jumps back into the context of an InputManager so the value can be
read from a GMenu2X object's configuration.

A GMenu2X object is also passed to InputManager::init.