dinput: Implement DIPROP_KEYNAME property for keyboard device

[Sergey Khodych]

> No, there are 2 separate implementations - one ascii another unicode. During 
> the object creation one would specify either ascii or unicode RIID.

Objects IDirectInputDevice8W and IDirectInputDevice8A has function
GetProperty. This function get struct of type DIPROPSTRING as
input/output parameter in our case . This struct has output field wsz of
type WCHAR[ MAX_PATH ] and other input fields not depends of charset.
So type of returned data will be WCHAR[ MAX_PATH ] in both case.
So objects IDirectInputDevice8W and IDirectInputDevice8A has the same
implementation of function GetProperty and this implementation is for
Unicode in both case.