First part of code to allow configurable hotkeys.

First part of code to allow configurable hotkeys.
include/configuration.h, src/misc/configuration.h:
 * added a CONFIG_ITEM_KEY
include/vlc_keys.h:
 * constants to identify keys
src/libvlc.h:
 * provide a set of hotkey config options
modules/gui/wxwindows/interface.cpp, wxwindows.h:
 * set the configured hotkeys as accelerator for the appropriate menus
modules/gui/wxwindows/preferences.cpp:
 * start to support CONFIG_ITEM_KEY. Options are showed with the right value
   but any changes made have no effect yet.