Move where the project ID is stored to a more general preference node

Currently we save the project ID under the `c.g.c.t.e.appengine.deploy` preference node, which is fine as long as it is only used for deployment. But later we (most probably) will have features that need a project ID but has nothing to do with deploy or appengine, e.g. deploy to GCE. Therefore it's better to store the project ID associated with the Eclipse project in the `com.google.cloud.tools.eclipse` preference node (or a similar, general node).