Plain text editor: Make sure user setting about folding markers is used

It was being reset based on the highlighter definitions.

Consider some refactoring with the related options as well to avoid
overriding user's settings.

Done-with: Thorbjorn Lindeijer

src/plugins/texteditor/basetexteditor.h

@@ -207,7 +207,7 @@ public:
     void setLineSeparatorsAllowed(bool b);
     bool lineSeparatorsAllowed() const;
 
-    void setCodeFoldingVisible(bool b);
+    void updateCodeFoldingVisible();
     bool codeFoldingVisible() const;
 
     void setCodeFoldingSupported(bool b);