pan_tool: add possibility to Pan using keys
pan_tool: add possibility to Pan using keys

ZoomTool has keys support, why not PanTool?

Still disabled by default
add pan_keys_step=50 to constructor to enable it.

Signed-off-by: Pierre Tardy <bb5a2a16eebb4a48138fca46fede24b3c8d635fe@intel.com>

burnpanck/chaco,burnpanck/chaco,ContinuumIO/chaco,ContinuumIO/chaco,burnpanck/chaco,tommy-u/chaco,tommy-u/chaco,tommy-u/chaco,ContinuumIO/chaco,ContinuumIO/chaco