Rewrite GdkWin32Keymap (load table directly from layout DLL)

The old code used repeated calls to `ToUnicodeEx` to populate
the translation table, which is slow and buggy. The new code
directly loads the layout driver DLLs from Windows.

Associated issues: #2055 #1033
Merge request: !1051

GdkWin32Keymap cleanup

Conform to C89, improve comments, whitespace

gdk/win32/meson.build

@@ -13,6 +13,8 @@ gdk_win32_sources = files(
   'gdkglcontext-win32.c',
   'gdkglobals-win32.c',
   'gdkkeys-win32.c',
+  'gdkkeys-win32-impl.c',
+  'gdkkeys-win32-impl-wow64.c',
   'gdkmain-win32.c',
   'gdkmonitor-win32.c',
   'gdkproperty-win32.c',