This operator is to ':' what [*i..j] is to ';'.

Part of issue #51.

* doc/tl/tl.tex: Document syntax, semantic, and trivial
simplifications.
* doc/tl/spotltl.sty: Add macros for new operators.
* src/ltlast/bunop.cc, src/ltlast/bunop.hh: Implement it.
* src/ltlast/multop.cc: Add some trivial simplifications.
* src/ltlparse/ltlparse.yy, src/ltlparse/ltlscan.ll: Parse it.
* src/ltltest/equals.test, src/ltltest/latex.test,
src/tgbatest/ltl2tgba.test: Add more tests.
* src/ltlvisit/randomltl.cc: Output this operator in
random PSL formulas.
* src/ltltest/rand.test: Adjust.
* src/tgbaalgos/ltl2tgba_fm.cc: Add translation rules.
* src/ltlvisit/tostring.cc: Add pretty printing code.
* src/ltlvisit/simplify.cc, src/ltlvisit/snf.cc: Adjust
switches.
* NEWS: Mention the new operator.