tl: add support for X[n], F[n:m] and G[n:m]

* NEWS, doc/tl/tl.tex, doc/tl/tl.bib: Document these new operators.
* spot/parsetl/parsetl.yy, spot/parsetl/scantl.ll: Parse those.
* spot/tl/formula.cc, spot/tl/formula.hh: Add constructors.
* spot/gen/formulas.cc: Use it.
* tests/core/sugar.test: New file.
* tests/Makefile.am: Add it.

doc/tl/tl.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
@@ -157,6 +158,17 @@
 		  about this problem.}
 }
 
+@InProceedings{	  jacobs.16.synt,
+  author	= {Swen Jacobs and Felix Klein and Sebastian Schirmer},
+  title		= {A High-Level {LTL} Synthesis Format: {TLSF} v1.1},
+  booktitle	= {Proceedings Fifth Workshop on Synthesis (SYNT@CAV'16)},
+  pages		= {112--132},
+  year		= {2016},
+  series	= {Electronic Proceedings in Theoretical Computer Science},
+  volume	= {229},
+  doi		= {10.4204/EPTCS.229.10}
+}
+
 @InProceedings{	  manna.87.podc,
   author	= {Zohar Manna and Amir Pnueli},
   title		= {A hierarchy of temporal properties},