Work around the removal of PsiQualifiedNamedElement.getContainer()

[bjansen]

https://upsource.jetbrains.com/idea-ce/revision/idea-ce-f0bb0c69d64bfa2851995fa82f44ec5edc45d04e

This breaks in 2016.2.1 EAP when I call "go to class".

com.intellij.psi.impl.compiled.ClsClassImpl.getContainer()Lcom/intellij/psi/PsiQualifiedNamedElement;
java.lang.NoSuchMethodError: com.intellij.psi.impl.compiled.ClsClassImpl.getContainer()Lcom/intellij/psi/PsiQualifiedNamedElement;
    at org.intellij.plugins.ceylon.ide.compiled.CeylonClassDecorator$1.getLocationString(CeylonClassDecorator.java:117)
    at com.intellij.ide.util.NavigationItemListCellRenderer$LeftRenderer.customizeCellRenderer(NavigationItemListCellRenderer.java:180)
    at com.intellij.ui.ColoredListCellRenderer.getListCellRendererComponent(ColoredListCellRenderer.java:76)
    at com.intellij.ide.util.NavigationItemListCellRenderer.getListCellRendererComponent(NavigationItemListCellRenderer.java:69)
    at com.intellij.ide.util.gotoByName.ContributorsBasedGotoByModel$1.getListCellRendererComponent(ContributorsBasedGotoByModel.java:76)