context menu should not be available on closed projects

At least the "Create Formatter profile" menu throws an exception when invoked on a closed project:
```
java.lang.Exception: Project 'checkstyle' is not open.
at org.eclipse.core.internal.resources.ResourceException.provideStackTrace(ResourceException.java:42)
at org.eclipse.core.internal.resources.ResourceException.(ResourceException.java:38)
at org.eclipse.core.internal.resources.Project.checkAccessible(Project.java:153)
at org.eclipse.core.internal.resources.File.create(File.java:112)
at org.eclipse.core.internal.resources.File.create(File.java:181)
at net.sf.eclipsecs.core.transformer.FormatterConfigWriter.createOrUpdateFile(FormatterConfigWriter.java:114)
at net.sf.eclipsecs.core.transformer.FormatterConfigWriter.writeFormatterSettings(FormatterConfigWriter.java:103)
at net.sf.eclipsecs.core.transformer.FormatterConfigWriter.writeSettings(FormatterConfigWriter.java:72)
at net.sf.eclipsecs.core.transformer.FormatterConfigWriter.(FormatterConfigWriter.java:64)
at net.sf.eclipsecs.core.transformer.CheckstyleTransformer.transformRules(CheckstyleTransformer.java:124)
at net.sf.eclipsecs.core.jobs.TransformCheckstyleRulesJob.runInWorkspace(TransformCheckstyleRulesJob.java:116)
at org.eclipse.core.internal.resources.InternalWorkspaceJob.run(InternalWorkspaceJob.java:43)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:63)
```