IE C++: listen for document.onfocusin to catch browser window focus events.

- previously when moving between browser windows in IE, no events would
  be fired and the active tab would not be updated in the COM servers,
  thus making FG <=> BG messages non-functional.
- this commit installs an event handler for the document.onfocusin event
  that catches incoming focus events and updates the active tab
  accordingly.
- also, care needs to be taken to detach the previous document before
  adding the listener to the new document since ATL will assert when
  attempting to DispEventAdvise on a new document before unadvising.