W32: Fix for commit 1f74f12d9, re-enabling decimal separator key

W32: Fix for commit 1f74f12d9, re-enabling decimal separator key

1f74f12d9 rendered entry of keypad decimal mark unuseable for
several national keyboard layouts, this commit amends that, at
least for W32, and makes GTK+ behave more or less the same way
W32 behaves.

The patch works like this:
- When typing the first character at the keyboard or when switching
  keyboard layouts, the decimal mark character will be cached in the
  static variable "decimal_mark" within gdkkeys-win32.c

- in case of WIN32, gdk_keyval_to_unicode() asks gdkkeys-win32.c for the
  current decimal_mark when converting GDK_KEY_KP_Decimal.

https://bugzilla.gnome.org/show_bug.cgi?id=756751