Update Manager now shows on startup (resolves #2557)

This PR is to fix [#2557](https://github.com/processing/processing/issues/2557).

What was happening was that the previous hasUpdates() used allContributions, which was of type ArrayList<Contribution>, but actually consisted of a list of AvailableContribution objects, and AvailableContribution.isInstalled() always returns false, which means that the Update Manager would never show because it thought that there was nothing installed.  
Now I have used methods in Base and Editor which accurately give all the libs/tools/modes installed.  
The checking for updates itself works well without any changes, and needs no modification.