Create unified key mappings for all platforms

[GoogleCodeExporter]

Currently in examples key presses that are handled in 
KeyboardHandler.OnKeyEvent, are checked against raw integer values to check for 
key code. The code for Linux and Windows is as follows:

    # F5
    if (linux and event["native_key_code"] == 71) \
            or (windows and event["windows_key_code"] == 116):
        print("[wxpython.py] F5 pressed, calling"
                " browser.ReloadIgnoreCache()")
        browser.ReloadIgnoreCache()
        return True
    # Escape
    if (linux and event["native_key_code"] == 9) \
            or (windows and event["windows_key_code"] == 27):
        print("[wxpython.py] Esc pressed, calling browser.StopLoad()")
        browser.StopLoad()
        return True
    # F12
    if (linux and event["native_key_code"] == 96) \
            or (windows and event["windows_key_code"] == 123):
        print("[wxpython.py] F12 pressed, calling"
                " browser.ShowDevTools()")
        browser.ShowDevTools()
        return True

The code for Mac is as follows:

    # Cmd+Opt+I
    if event["modifiers"] == 136 and event["character"] == 94:
        browser.ShowDevTools()
        return True
    # F5
    if event["modifiers"] == 0 and event["character"] == 63240:
        browser.ReloadIgnoreCache()
        return True
    # Esc
    if event["modifiers"] == 0 and event["character"] == 27:
        browser.StopLoad()

Actually in this case "123" key code could be replaced with cefpython.VK_F12 on 
Windows. Virtual key codes are provided only for Windows, see the VirtualKey 
wiki page. We should provide similar key codes using VK_ constants for other 
platforms: Mac, Linux.

On Mac and Linux some key codes are similar (Esc), but others like F5 are 
different.

In the kivy_.py example there is a mapping of Linux keys that could be used to 
translate Windows VK key codes to Linux ones. See the translate_to_cef_keycode 
function: 
https://code.google.com/p/cefpython/source/browse/cefpython/cef3/linux/binaries_
64bit/kivy_.py?r=ad99514963ae#355

In the code pasted above, you can see that on all platforms a different 
event[key] is being checked:
 * On Windows it is event["windows_key_code"]
 * On Linux it is event["native_key_code"]
 * On Mac it is event["character"]

I think we should add event["virtual_key"] to simplify things. This virtual_key 
would be checked against cefpython.VK_ constants.

Maybe we should also export all VK_ to a dict? And make it accessible through 
cefpython.virtual_keys? I think this could be useful, for example:

  {
    "a": 65,
    "b": 66,
    ...
  }

Original issue reported on code.google.com by czarek.t...@gmail.com on 21 Jan 2015 at 9:04