Move some parts of window.c into putty.c.

This prepares the ground for a second essentially similarly-shaped
program reusing most of window.c but handling its command line and
startup differently. A couple of large parts of WinMain() to do with
backend selection and command-line handling are now subfunctions in a
separate file putty.c.

Also, our custom AppUserModelId is defined in that file, so that it
can vary with the client application.

windows/jump-list.c

@@ -735,7 +735,8 @@ bool set_explicit_app_user_model_id(void)
 
     if (p_SetCurrentProcessExplicitAppUserModelID)
     {
-        if (p_SetCurrentProcessExplicitAppUserModelID(L"SimonTatham.PuTTY") == S_OK)
+        const wchar_t *id = get_app_user_model_id();
+        if (p_SetCurrentProcessExplicitAppUserModelID(id) == S_OK)
         {
           return true;
         }