Merge topic 'genex-list-sort-comparator'

4d86ef5e86 GenEx: add $<LIST:SORT,...,COMPARATOR,body> comparator
c418a3dc85 GenEx: factor out reusable SORT comparator and option-parsing helpers
e335872625 GenEx: generalize bound operands to a frame and add $<_1>

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Merge-request: !12228