if e is pure eventuality and g => e, then e U g = Fg
if u is purely universal and u => g, then u R g = Gg
Fixes#93.
* doc/tl/tl.tex, NEWS: Document the rules.
* spot/tl/simplify.cc: Implement them.
* tests/core/reduccmp.test: Test them.
* tests/core/det.test: Adjust.
* spot/twa/twa.hh (twa::get_or_set_named_prop): New method.
* spot/twaalgos/emptiness.cc, spot/twaalgos/isdet.cc: Use it to not
overwrite existing highlights.
* tests/core/det.test: Add test case.