Lemur version 1.13.0

Multi-gamepad support, tweaks to Popup effect management, other minor updates.

Change log:

• Added constants EFFECT_OPEN and EFFECT_CLOSE to Panel to make it easier
  to standardize open/close effects.
• Modified PopupState to use the new Panel.EFFECT_OPEN and Panel.EFFECT_CLOSE
  instead of string literals.
• Added PopupState.getGuiSize() that returns the (potentially scaled) screen size
  based on the PopupState's guiNode.
• Modified PopupState to properly request focus of the things it is popping up.
  Without this, keyboard navigation was trickier.
• Modified InputMapper to map all gamepad buttons and axes, even if they don't
  match up with predefined constants.
• Modified Axis to include constants for JOYSTICK_LEFT_TRIGGER and JOYSTICK_RIGHT_TRIGGER
  along with support in InputMapper.
• Added support for multiple gamepads/joysticks. There is now a new InputDevice
  class that provides gamepad-specific versions of the regular Button/Axis
  constants. Support for non-joystick-specific mappings still works like before.
• Modified DragHandler to have set/getConsumeDrags() and set/getConsumeDrops() for
  tweaking the internal boolean settings.
• Added GuiGlobals.releaseFocus() and the corresponding FocusManagerState.releaseFocus()
  which are safe ways to clear the focus for a currently focused element if it is
  still focused. Useful for conditionally clearing focus when tearing down a window.
• Modified PopupState to call releaseFocus() for the popup when it is closing.