activate the window when needed, add a menu, bugfixed

• support to activate the window when run with file association
• support to activate the window when image file dropped
• add 'Divide wide image into 2 pages'
• Known bug:
  • Since the rotation of the tablet can not be detected, correct display can not be performed immediately after rotation.
  • You can solve by moving to the next page and returning to the original page
• bugfixed: When switching '2 page spread', the page has changed