Merge topic 'genex-list-transform-apply'

0f6e8ded1d GenEx: add $<LIST:TRANSFORM,...,APPLY,body> action
c0a0b7fdd9 GenEx: add bound-operand binding mechanism and $<_0>
caa51f5689 GenEx: factor TRANSFORM selector parsing and selection

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Merge-request: !12203