formula: track Δ₁, Σ₂, Π₂, and Δ₂ membership

* spot/tl/formula.hh, spot/tl/formula.cc: Update the properties
and track them.
* tests/core/kind.test: Augment the test case.
* doc/tl/tl.tex, doc/spot.bib, NEWS: Document these new classes.

doc/spot.bib

@@ -1,3 +1,4 @@
+
 @InProceedings{	  babiak.12.tacas,
   author	= {Tom{\'a}{\v{s}} Babiak and Mojm{\'i}r
 		  K{\v{r}}et{\'i}nsk{\'y} and Vojt{\v{e}}ch {\v{R}}eh{\'a}k
@@ -470,6 +471,20 @@
   doi		= {10.1145/3209108.3209161}
 }
 
+@Article{	  esparza.24.acm,
+  author	= {Javier Esparza and Rub\'{e}n Rubio and Salomon Sickert},
+  title		= {Efficient Normalization of Linear Temporal Logic},
+  year		= 2024,
+  publisher	= {Association for Computing Machinery},
+  address	= {New York, NY, USA},
+  volume	= {71},
+  number	= {2},
+  issn		= {0004-5411},
+  doi		= {10.1145/3651152},
+  journal	= {Journal of the ACM},
+  month		= apr
+}
+
 @InProceedings{	  etessami.00.concur,
   author	= {Kousha Etessami and Gerard J. Holzmann},
   title		= {Optimizing {B\"u}chi Automata},