Support for tab orders on code assist

Takes TextMate snippet syntax that is supported
by VS Code to mark tab positions for method parameter
completions.

server/org.jboss.tools.vscode.java/src/org/jboss/tools/vscode/java/CompletionProposalReplacementProvider.java

@@ -197,8 +197,9 @@ private void appendGuessingCompletion(StringBuilder buffer, CompletionProposal p
 
 			positions.add(buffer.length());
 			positions.add(argument.length);
-
+			buffer.append("{{");
 			buffer.append(argument);
+			buffer.append("}}");
 		}
 	}