Optional Content (OC) radiobutton (RB) groups implemented. Resolves #18823.

The code parses the /RBGroups entry in the OC configuration dict and adds the property `rbGroups' to instances of the OptionalContentGroup class. rbGroups takes an array of Sets, where each Set instance represents an RB group the OptionalContentGroup instance is a member of. Such a Set instance contains all OCG ids within the corresponding RB group. RB groups an OCG is associated with are processed when its visibility is set to true, as required by the PDF spec.

test/pdfs/.gitignore

@@ -589,6 +589,7 @@
 !issue15690.pdf
 !bug1802888.pdf
 !issue15759.pdf
+!issue18823.pdf
 !issue15753.pdf
 !issue15789.pdf
 !fields_order.pdf

test/test_manifest.json

@@ -6875,6 +6875,23 @@
       "7R": false
     }
   },
+  {
+    "id": "issue18823-default",
+    "file": "pdfs/issue18823.pdf",
+    "md5": "f5246c476516c96df106ced0c5839da3",
+    "rounds": 1,
+    "type": "eq"
+  },
+  {
+    "id": "issue18823-group-three",
+    "file": "pdfs/issue18823.pdf",
+    "md5": "f5246c476516c96df106ced0c5839da3",
+    "rounds": 1,
+    "type": "eq",
+    "optionalContent": {
+      "15R": true
+    }
+  },
   {
     "id": "issue2829",
     "file": "pdfs/issue2829.pdf",