tmf: Keep usage of ViewerSorter in navigatorContent extension point

tmf: Keep usage of ViewerSorter in navigatorContent extension point

Reverting the change to TmfViewerSorter from commit e707a55.

Platform UI doesn't support ViewerComparator in the
org.eclipse.ui.navigator.navigatorContent extension point in the
navigatorContent/commonSorter element (refer to Bug 484248).

This causes a ClassCastException and prevents the TMF project model
elements from being sorted properly.

Change-Id: I3d63032763dc435b594c3f9e8df04d0cb6b38609
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/62640
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>