Minor improvements to the Contribution Manager's updates check

The commit of this PR does the following:

* Makes the 'Contribution updates are available' popup less obtrusive- displays the pop-up only if none of the Contributions Manager windows have been opened yet, but at least one installed contribution has an update: resolves [#2859](https://github.com/processing/processing/issues/2859).
* Adds examples-packages into the update check