Support multiple comparators (for sorting) in outline view.

At the moment, the outline view only supports a single comparator. The default sort contribution is defined in org.eclipse.xtext.ui.editor.outline.actions.SortOutlineContribution.

Simply registering multiple sort contributions does not work since the underlying API does not support multiple comparators, cf. org.eclipse.xtext.ui.editor.outline.impl.OutlineFilterAndSorter (multiple filters via add, only one comparator via set).

If would be great if multiple comparators were supported, e.g., by extending the comparator interface org.eclipse.xtext.ui.editor.outline.impl.OutlineFilterAndSorter.IComparator with a org.eclipse.xtext.ui.editor.actions.AbstractToggleActionContributor.configureAction(Action) like method. Then, instead of providing different "sort contributions", one can configure (in the UIModule) multiple comparators.

Of course in that case the "multiple comparator aware" sort contributions would have to toggle between the comparators.

Cf. forum: https://www.eclipse.org/forums/index.php/mv/msg/1088234/1770617/

Read org.eclipse.xtext.ui.editor.outline.impl.OutlineFilterAndSorter and the IComparator interface, then inspect org.eclipse.xtext.ui.editor.outline.actions.SortOutlineContribution and the related UIModule configuration. Define how multiple comparators are configured and selected, including how sort contributions toggle them; done means the outline view supports multiple registered comparators without breaking existing sorting.