Regression in recent fix for T66088 [0]. caused by much older problem
introduced by [1] & [2].

Unlike other platforms, as of [1] GHOST/Win32 was keeping track of the
pressed modifier keys, which were cleared on window activation [2].

Since GHOST/Win32 would clear it's modifier state on window activation
and only changes to modifier state would generate key events, activating
the window and releasing the modifier would not send the release event.

Resolve this by removing the modifier state from GHOST/Win32,
passing modifier press/release events through to Blender
(matching other GHOST back-ends). Instead, use key-repeat detection
to prevent repeated modifier keys from being generated - an alternate
solution to T26446.

[0]: 8bc76bf4b957c51ddc5a13c6305f05c64b218a27
[1]: d6b43fed313b60bb6a269680b3c5622955b8a690
[2]: 6b987910e43ff5f91512a3c361ea3141590d4e45