use a bibtex file to collect all references in Doxygen

* doc/tl/tl.bib: Move ...
* doc/spot.bib: ... here, and augment it with all references that
appeared verbatim in Doxygen comments.
* doc/Makefile.am, doc/tl/Makefile.am
doc/tl/tl.tex: Adjust for the move.
* doc/Doxyfile.in: Point to spot.bib.
* spot/gen/automata.hh, spot/gen/formulas.hh, spot/misc/game.hh,
spot/misc/minato.hh spot/taalgos/emptinessta.hh,
spot/taalgos/minimize.hh, spot/taalgos/tgba2ta.hh, spot/tl/formula.hh,
spot/tl/remove_x.hh, spot/tl/simplify.hh, spot/tl/snf.hh,
spot/twaalgos/cobuchi.hh, spot/twaalgos/cycles.hh,
spot/twaalgos/dualize.hh, spot/twaalgos/gtec/gtec.hh,
spot/twaalgos/gv04.hh, spot/twaalgos/ltl2taa.hh,
spot/twaalgos/ltl2tgba_fm.hh, spot/twaalgos/magic.hh,
spot/twaalgos/minimize.hh, spot/twaalgos/parity.hh,
spot/twaalgos/powerset.hh, spot/twaalgos/randomgraph.hh,
spot/twaalgos/se05.hh, spot/twaalgos/simulation.hh,
spot/twaalgos/strength.hh, spot/twaalgos/stutter.hh,
spot/twaalgos/tau03.hh, spot/twaalgos/totgba.hh,
spot/twaalgos/toweak.hh: Use \cite instead of a verbatim bibtex entry.

spot/twaalgos/cycles.hh

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2012, 2013, 2014, 2015, 2018 Laboratoire de Recherche et
+// Copyright (C) 2012-2015, 2018-2019 Laboratoire de Recherche et
 // Développement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -28,20 +28,7 @@ namespace spot
   /// \brief Enumerate elementary cycles in a SCC.
   ///
   /// This class implements a non-recursive version of the algorithm
-  /// on page 170 of:
-  /** \verbatim
-      @Article{loizou.82.is,
-        author =  {George Loizou and Peter Thanisch},
-        title =   {Enumerating the Cycles of a Digraph: A New
-                   Preprocessing Strategy},
-        journal = {Information Sciences},
-        year =           {1982},
-        volume =  {27},
-        number =  {3},
-        pages =   {163--182},
-        month =   aug
-      }
-      \endverbatim */
+  /// on page 170 of \cite loizou.82.is .
   /// (the additional preprocessings described later in that paper are
   /// not implemented).
   ///