Added code to load and enable xf86 keysyms

[krysnox]

Related to #443

Added

# Load XF86 keysym defs for media function keys
try:
    Xlib.XK.load_keysym_group("xf86")
except err:
    log.warn("Xlib backend needs python-xlib 0.15rc1 or higher\n")

in _pyautogui_x11.py to load xf86 keysyms for using some multimedia player keys in Linux. I can edit it if there is more efficient way to use it.

Also added some keys in keyboardMapping which I used in a project.

    'volumedown':        _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_AudioLowerVolume')),
    'volumeup':          _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_AudioRaiseVolume')),
    'playpause':         _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_AudioPlay')),
    'nexttrack':         _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_AudioNext')),
    'prevtrack':         _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_AudioPrev')),

I will be adding more such keys as I will know their functions.

[domstoppable]

I was just about to make a pull request that's nearly identical to this one. I believe a more complete list includes these keys:

    'browserback':       _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_Back')),
    'browserforward':    _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_Forward')),
    'browserrefresh':    _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_Refresh')),
    'browserstop':       _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_Stop')),
    'browsersearch':     _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_Search')),
    'browserfavorites':  _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_Favorites')),
    'browserhome':       _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_HomePage')),
    'volumemute':        _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_AudioMute')),
    'volumedown':        _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_AudioLowerVolume')),
    'volumeup':          _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_AudioRaiseVolume')),
    'nexttrack':         _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_AudioNext')),
    'prevtrack':         _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_AudioPrev')),
    'stop':              _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_AudioStop')),
    'playpause':         _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_AudioPlay')),
    'launchmail':        _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_Mail')),
    'launchmediaselect': _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_AudioMedia')),
    'launchapp1':        _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_Launch0')),
    'launchapp2':        _display.keysym_to_keycode(Xlib.XK.string_to_keysym('XF86_Launch1')),

[krysnox]

@domstoppable yes, yours is more complete. I have also added some more keys. We can merge our branches.