Configurable keybindings for debugging nxagent

Some keystrokes are only available in special debug builds of nxagent
and will be ignored otherwise. These are

debug_tree
  Show the window trees of both internal and external
  windows. Included if DEBUG_TREE is defined.

regions_on_screen
  Make corrupted regions visible. Included if DUMP is defined.

test_input
  Activate/deactive input device debugging. Included if NX_DEBUG_INPUT
  is defined.

deactivate_input_devices_grab
  Release grab of input devices.  Included if NX_DEBUG_INPUT is
  defined.

They can be configured by adding these lines to keystrokes.cfg, below
keystrokes represent the default:

<keystroke action="debug_tree" Control="1" AltMeta="1" key="q" />
<keystroke action="regions_on_screen" Control="1" AltMeta="1" key="a" />
<keystroke action="test_input" Control="1" AltMeta="1" key="x" />
<keystroke action="deactivate_input_devices_grab" Control="1" AltMeta="1" key="y" />
