Refactor project-type expression tests

Our `com.google.cloud.tools.eclipse.appengine.onlyInGCPProjects` only responds `true` to objects that can be adapted to an `IProject`. If the user clicks on a file, for example, then `onlyInGCPProjects == false` as an `IFile` cannot be adapted to an `IProject`. As it can be useful to check if a resource is in a GCP project (e.g., for the launch functionality), it would make sense for this to be called `isGCPProject` and add an `isOrIsinGCPProject`. And perhaps add additional tests for App Engine projects, and specifically for Standard and Flexible.

The underlying WTP `org.eclipse.wst.common.project.facet.core.projectFacet` test does try to adapt the incoming object to an `IResource` and checks its containing project.