Allow giving TextEditorFactory custom CommentDefinition

It was not possible to set custom comment styles.
Also simplifies the code for the predefined styles.

Change-Id: Id7f345d65b747bfac5a15e3eb15cd2beb106b281
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/cppeditorplugin.cpp

@@ -95,7 +95,7 @@ public:
         setEditorWidgetCreator([]() { return new CppEditorWidget; });
         setEditorCreator([]() { return new CppEditor; });
         setAutoCompleterCreator([]() { return new CppAutoCompleter; });
-        setCommentStyle(CommentDefinition::CppStyle);
+        setCommentDefinition(CommentDefinition::CppStyle);
         setCodeFoldingSupported(true);
         setMarksVisible(true);
         setParenthesesMatchingEnabled(true);