IE BHO: Improve robustness of DOM attachment and script/style injection.

- page refresh events were not causing scripts/styles and DOM attachment
  to occur. This commit makes the logic more robust to add this support.
- 3 events from the WebBrowser2 object now can trigger DOM attachment
  and script/style injection: NavigateComplete2, DocumentComplete and
  DownloadComplete.
- DOM attachment and script/styles are now idempotent -- once the COM
  servers and scripts/styles are injected into a window DOM object,
  these operations will not occur again until the window DOM object
  is reloaded or the browser navigates to a new page.
- Idempotency is achieved by inspecting the propertys on the IHTMLWindow
  object -- after script/style injection, a property is added to the
  window to record the completion of this operation.