Don't show code folding margin when it isn't supported

Introduce setCodeFoldingSupported to the BaseTextEditor, and only
enabled it for the C++ and QtScript editors. This removes the useless
margin meant for code folding markers from the plain text editor and
.pro file editor.

src/plugins/cppeditor/cppeditor.cpp

@@ -185,6 +185,7 @@ CPPEditor::CPPEditor(QWidget *parent)
 {
     setParenthesesMatchingEnabled(true);
     setMarksVisible(true);
+    setCodeFoldingSupported(true);
     setCodeFoldingVisible(true);
     baseTextDocument()->setSyntaxHighlighter(new CppHighlighter);
 //    new QShortcut(QKeySequence("Ctrl+Alt+M"), this, SLOT(foo()), 0, Qt::WidgetShortcut);