highlight: improve support for highlighted edges

* spot/twa/twa.cc, spot/twa/twa.hh: Add a way to
remove named properties.
* spot/twa/twagraph.cc: Clear highlight-edges on operations
that reorder the edge vector.
* spot/twaalgos/randomize.cc, spot/twaalgos/randomize.hh:
Preserve highlighted state, but not highlighted edges.
* spot/twaalgos/hoa.cc: Adjust output of highlight-edge
when the edges are not stored in order.
* tests/core/readsave.test, tests/core/tgbagraph.test,
tests/core/twagraph.cc: More test cases.

tests/core/tgbagraph.test

@@ -226,17 +226,34 @@ digraph G {
   node [shape="circle"]
   I [label="", style=invis, width=0]
   I -> 2
-  0 [label="s1"]
+  0 [label="s1", style="bold", color="#F15854"]
   1 [label="s2"]
-  2 [label="s3"]
+  2 [label="s3", style="bold", color="#4D4D4D"]
 }
 digraph G {
   rankdir=LR
   node [shape="circle"]
   I [label="", style=invis, width=0]
   I -> 0
-  0 [label="s3"]
+  0 [label="s3", style="bold", color="#4D4D4D"]
 }
+HOA: v1.1
+States: 3
+Start: 2
+AP: 0
+acc-name: all
+Acceptance: 0 t
+properties: trans-labels explicit-labels state-acc complete
+properties: deterministic
+spot.highlight.edges: 3 1 2 2 1 3
+--BODY--
+State: 0
+[t] 0
+State: 1
+[t] 0
+State: 2
+[t] 1
+--END--
 EOF
 
 diff stdout expected