(Vwin32_mouse_move_interval): New lisp variable.

(Vwin32_mouse_move_interval): New lisp variable.
(syms_of_win32fns): Add Vwin32_mouse_move_interval to syms.
(saved_mouse_msg): Renamed to saved_mouse_button_msg.
(timer_id): Renamed to mouse_button_timer.
(saved_mouse_move_msg, mouse_move_timer): New variables.
(win_msg_worker): Delete WM_TIMER code.
(win32_wnd_proc): Handle WM_TIMER events here. Use separate timers
for mouse down and mouse move (including scroll bar drag) events.
Add new handling code for WM_VSCROLL and WM_MOUSEMOVE events.
Only filter WM_MOUSEMOVE events when a button is held down.
Always pass on message to DefWindowProc after calling
TranslateMessage.  Reset keyboard modifiers when losing focus.

(win32_wnd_proc): When passing modifier keystrokes back
to Windows, invoke TranslateMessage on them.