Windows: move GUI timer handling into a utils module.

Previously, the timing.c subsystem worked in Windows PuTTY by means of
scheduling WM_TIMER messages to be sent to the terminal window. Now it
uses a separate hidden window instead, and all the machinery for
handling that window lives on its own in windows/utils/gui-timing.c.

Most immediately, this removes a use of wgs.term_hwnd that will become
awkward when I move that structure in a following commit. But also, it
will make it easier to add the same timing subsystem to unrelated GUI
programs, such as Windows Pageant: if we ever decide to implement
automatic deletion or re-encryption of unused keys after a timeout,
this will help make that easier.

windows/utils/gui-timing.c

@@ -0,0 +1,56 @@
+#include "putty.h"
+
+#define TIMING_CLASS_NAME "PuTTYTimerWindow"
+#define TIMING_TIMER_ID 1234
+static long timing_next_time;
+static HWND timing_hwnd;
+
+static LRESULT CALLBACK TimingWndProc(HWND hwnd, UINT message,
+                                      WPARAM wParam, LPARAM lParam)
+{
+    switch (message) {
+      case WM_TIMER:
+        if ((UINT_PTR)wParam == TIMING_TIMER_ID) {
+            unsigned long next;
+
+            KillTimer(hwnd, TIMING_TIMER_ID);
+            if (run_timers(timing_next_time, &next)) {
+                timer_change_notify(next);
+            } else {
+            }
+        }
+        return 0;
+    }
+    return DefWindowProc(hwnd, message, wParam, lParam);
+}
+
+void setup_gui_timing(void)
+{
+    WNDCLASS wndclass;
+
+    memset(&wndclass, 0, sizeof(wndclass));
+    wndclass.lpfnWndProc = TimingWndProc;
+    wndclass.hInstance = hinst;
+    wndclass.lpszClassName = TIMING_CLASS_NAME;
+
+    RegisterClass(&wndclass);
+
+    timing_hwnd = CreateWindow(
+        TIMING_CLASS_NAME, "PuTTY: hidden timing window",
+        WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT,
+        100, 100, NULL, NULL, hinst, NULL);
+    ShowWindow(timing_hwnd, SW_HIDE);
+}
+
+void timer_change_notify(unsigned long next)
+{
+    unsigned long now = GETTICKCOUNT();
+    long ticks;
+    if (now - next < INT_MAX)
+        ticks = 0;
+    else
+        ticks = next - now;
+    KillTimer(timing_hwnd, TIMING_TIMER_ID);
+    SetTimer(timing_hwnd, TIMING_TIMER_ID, ticks, NULL);
+    timing_next_time = next;
+}