new option: on-screen-remove-when-edit, decrease overlay priority

- Make overlay removing when changing a buffer customizable (new option
  `on-screen-remove-when-edit').  Off by default (previously, the
  feature was always enabled).

- Radically decreased on-screen-overlay-priority (to 30).  It should be
  less than isearch and ediff overlay priorities, but still larger than
  e.g. stripe-buffer overlay priority.