Utils: Make injected default dialog parent better re-usable

... and use it for CheckableMessageBox. This enables using
the proper parent e.g. in upcoming changes to ExtensionSystem.

Change-Id: Ib0e52cbd3e73dc628955918e3f443ff65cccb189
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/fileutils.h

@@ -89,8 +89,6 @@ public:
     static FilePaths usefulExtraSearchPaths();
 
 #ifdef QT_WIDGETS_LIB
-    static void setDialogParentGetter(const std::function<QWidget *()> &getter);
-
     static bool hasNativeFileDialog();
 
     static FilePath getOpenFilePath(QWidget *parent,
@@ -236,7 +234,5 @@ private:
 
 QTCREATOR_UTILS_EXPORT QTextStream &operator<<(QTextStream &s, const FilePath &fn);
 
-bool isRelativePathHelper(const QString &path, OsType osType);
-
 } // namespace Utils