Windows tools: assorted '-demo' options.

Using a new screenshot-taking module I just added in windows/utils,
these new options allow me to start up one of the tools with
demonstration window contents and automatically save a .BMP screenshot
to disk. This will allow me to keep essentially the same set of demo
images and update them easily to keep pace with the current appearance
of the real tools as PuTTY - and Windows itself - both evolve.

windows/window.c

@@ -816,6 +816,8 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
     term_set_focus(term, GetForegroundWindow() == wgs.term_hwnd);
     UpdateWindow(wgs.term_hwnd);
 
+    gui_terminal_ready(wgs.term_hwnd, &wgs.seat, backend);
+
     while (1) {
         int n;
         DWORD timeout;