Fix issues with the Windows keyboard input code

[drmfinlay]

This PR fixes a few bugs with the Windows keyboard input code and cleans it up a bit. The changes are as follows:

• Fix a Windows keyboard input bug related to letter and number keys.
• Change the KeyboardInput struct to only translate virtual keycodes to scan codes when necessary (in __init__).
• Remove the hardcoded "soft" keys set and defer to the Win32 MapVirtualKey / MapVirtualKeyEx function instead, which returns 0 for virtual keycodes that don't have corresponding scancodes.
• Fix Windows keyboard input bug related to scan codes. The KeyboardInput struct in sendinput.py now falls back on the virtual-key code if there is no equivalent scan code available. This is not done if the virtual keycode is 0 (for KEYEVENTF_UNICODE mode).

These changes fix bugs that occur with different keyboard layouts and should make this implementation more reliable.

[drmfinlay]

@daanzu This might have fixed that weird bug (#182). If it didn't, then I can't see what else could be causing it. The input code is significantly more reliable now in any case.

I can say that this does fix a similar bug that occurs for me if I use my Dvorak keyboard layout and Dragonfly in 64-bit Python, which is what Kaldi requires.

[drmfinlay]

@Versatilus Just FYI, this removes the soft keys code you added into sendinput.py a little while ago. I have tested the SOFT_KEYS set with the MapVirtualKey() function and it returns 0 (meaning no scan code translation) for most of those keys. I assume the exceptions are due to layout/locale differences between us.

I'm happy to add it back in if this breaks something for you, but I'm pretty sure it will work fine.