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/misc/game.hh

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2017-2018 Laboratoire de Recherche et Développement
+// Copyright (C) 2017-2019 Laboratoire de Recherche et Développement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -93,19 +93,7 @@ public:
   typedef std::unordered_map<unsigned, unsigned> strategy_t;
 
   /// Compute the winning strategy and winning region of this game for player
-  /// 1 using Zielonka's recursive algorithm.
-  /** \verbatim
-      @article{ zielonka.98.tcs
-        title = "Infinite games on finitely coloured graphs with applications to
-        automata on infinite trees",
-        journal = "Theoretical Computer Science",
-        volume = "200",
-        number = "1",
-        pages = "135 - 183",
-        year = "1998",
-        author = "Wieslaw Zielonka",
-      }
-      \endverbatim */
+  /// 1 using Zielonka's recursive algorithm. \cite zielonka.98.tcs
   void solve(region_t (&w)[2], strategy_t (&s)[2]) const;
 
 private: