Windows: utility function to centre a window.

This was called from config box setup, and is obviously the kind of
thing that ought to be a reusable utility function.

windows/utils/centre_window.c

@@ -0,0 +1,20 @@
+/*
+ * Centre a window on the screen. Used to position the main config box.
+ */
+
+#include "putty.h"
+
+void centre_window(HWND win)
+{
+    RECT rd, rw;
+
+    if (!GetWindowRect(GetDesktopWindow(), &rd))
+        return;
+    if (!GetWindowRect(win, &rw))
+        return;
+
+    MoveWindow(win,
+               (rd.right + rd.left + rw.left - rw.right) / 2,
+               (rd.bottom + rd.top + rw.top - rw.bottom) / 2,
+               rw.right - rw.left, rw.bottom - rw.top, true);
+}