Part of issue #497.
* doc/org/concepts.org: Declare a new "aliases" named property.
* spot/parseaut/parseaut.yy: Fill the aliases named property.
* spot/twa/twa.cc (copy_named_properties_of): Copy it.
* spot/twaalgos/hoa.cc: Use "aliases" while encoding BDDs for
output.
* spot/twaalgos/hoa.hh: Add helper function to set/get aliases.
* python/spot/impl.i: Create a type for aliases.
* tests/core/parseaut.test: Adjust test case.
* tests/python/aliases.py: New file.
* tests/Makefile.am: Add it.
* NEWS: Mention this change.
