Uses the BuildStructureChanged event to extract available projects

@Warry, can you please take a look at this PR before it's getting merged.
The project drop-down list doesn't look good in Chrome on OSX (haven't tried FF) with this update.
Leaving the build page and coming back makes the browser render the list okay so it seems to be some UI trick that's needed.

Replacing these means you lose the subscribers; maybe you want to just reset their values not make new observables