Regression from f13fb4997f5e7849da26285938a3a0aea1769747.

One part of the regression was already fixed via
935b1dba41dd1e8f52a840c5057ae1eb925db159 (in `QuickFixPage`).

This change fixes remaining regressions that affect sort order of
markers where severity and priority attributes are involved.

Markers with higher values of severity/priority should be always sorted
first by default (the opposite to the "natural" sort order), because we
want that the *most important* markers are shown to the user by default
(without scrolling and filtering).

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/2845