Support GUI dialogs and message boxes better on MS-Windows

Support GUI dialogs and message boxes better on MS-Windows

* src/w32menu.c (TASKDIALOG_COMMON_BUTTON_FLAGS, TASKDIALOG_FLAGS)
(PFTASKDIALOGCALLBACK, TASKDIALOG_BUTTON, TASKDIALOGCONFIG)
(TDN_CREATED, TDM_ENABLE_BUTTON, TDF_ALLOW_DIALOG_CANCELLATION)
(TD_INFORMATION_ICON) [!MINGW_W64]: Define.
(TaskDialogIndirect_Proc): Declare function type.
(TASK_DIALOG_MAX_BUTTONS): New macro.
(task_dialog_callback): New callback function.
(w32_popup_dialog): Add dialog implementation using TaskDialog.
(globals_of_w32menu): Load TaskDialogIndirect from comctl32.dll.
(Bug#20481)

(cherry picked from commit aa7dee58d81817285208471074f1af598ebf0c98)