fix ClassCastException for New > Module/Package/File on Module node

OK, I'm not sure this is the right way to fix this, because this pull request may infringe upon https://github.com/ceylon/ceylon-ide-eclipse/issues/971

That being said, this is the problem : 
On a Module Node, New > Module / Package / Source File causes :
```
ava.lang.ClassCastException: com.redhat.ceylon.eclipse.code.navigator.SourceModuleNode cannot be cast to org.eclipse.core.runtime.IAdaptable
	at com.redhat.ceylon.eclipse.code.wizard.NewUnitWizardPage.getSelectedElement(NewUnitWizardPage.java:81)
	at com.redhat.ceylon.eclipse.code.wizard.NewUnitWizardPage.initFromSelection(NewUnitWizardPage.java:555)
	at com.redhat.ceylon.eclipse.code.wizard.NewUnitWizardPage.createControl(NewUnitWizardPage.java:94)
	at org.eclipse.jface.wizard.Wizard.createPageControls(Wizard.java:174)
	at org.eclipse.jface.wizard.WizardDialog.createPageControls(WizardDialog.java:736)
```