Be more systematic about user-interface timestamps.

Be more systematic about user-interface timestamps.

Before, the code sometimes used 'Time', sometimes 'unsigned long',
and sometimes 'EMACS_UINT', to represent these timestamps.  This
change causes it to use 'Time' uniformly, as that's what X uses.
This makes the code easier to follow, and makes it easier to catch
integer overflow bugs such as Bug#8664.
* frame.c (Fmouse_position, Fmouse_pixel_position):
Use Time, not unsigned long, for user-interface timestamps.
* keyboard.c (last_event_timestamp, kbd_buffer_get_event): Likewise.
(button_down_time, make_lispy_position, make_lispy_movement): Likewise.
* keyboard.h (last_event_timestamp): Likewise.
* menu.c (Fx_popup_menu) [!HAVE_X_WINDOWS]: Likewise.
* menu.h (xmenu_show): Likewise.
* term.c (term_mouse_position): Likewise.
* termhooks.h (struct input_event.timestamp): Likewise.
(struct terminal.mouse_position_hook): Likewise.
* xmenu.c (create_and_show_popup_menu, xmenu_show): Likewise.
* xterm.c (XTmouse_position, x_scroll_bar_report_motion): Likewise.
* systime.h (Time): New decl.  Pull it in from <X11/X.h> if
HAVE_X_WINDOWS, otherwise define it as unsigned long, which is
what it was before.
* menu.h, termhooks.h: Include "systime.h", for Time.