Add com.google.gdt.eclipse.suite.e42.feature to JBDS target platform to make building simpler

Discovered while documenting limitations & build instructions in JBDS-2456 that if we add com.google.gdt.eclipse.suite.e42.feature to the JBDS "multiple" target platform (or equivalently, if we we build using mvn clean install -Punified.target -Dtpc.group=jbosstools), we no longer need to include the -Pjboss-requirements-composite-extras-mirror profile to build JBDS, and can therefore build more simply like this:

mvn clean install -Pmaximum,multiple.target

So, I propose we add com.google.gdt.eclipse.suite.e42.feature to the JBDS TP so as to make building easier. The reason it's required is this dependency chain:

com.jboss.jbds.site.product depends on org.jboss.tools.maven.gwt depends on bundle com.google.gwt.eclipse.core which is included in com.google.gdt.eclipse.suite.e42.feature.

If you currently try to build using the JBDS TP and omit the -Pjboss-requirements-composite-extras-mirror profile flag, you get this:

[ERROR]   Software being installed: com.jboss.jbds.site.product raw:7.0.0.'SNAPSHOT'/format(n[.n=0;[.n=0;[-S]]]):7.0.0-SNAPSHOT
[ERROR]   Missing requirement: org.jboss.tools.maven.gwt 1.5.0.Alpha2-v20130412-2122-B29 requires 'bundle com.google.gwt.eclipse.core 2.5.0' but it could not be found

We could even go a step further and set multiple & maximum as defaults in the JBDS product root pom, so that a build need only be invoked with mvn clean install.