Fixes for monitorDOMChanges

- Dispatch pageModified event (to all frames in tab) on a DOM change.
- Clear translators for a tab before running any detection. In
  particular, ensure we don't clear valid translators for one frame
  when we run detection on another frame.
- Avoid re-creating Zotero.Translate instance when handling
  pageModified event. This also avoids dangling MutationObservers in
  a case where we have multiple MutationObservers and only one fires.