Fix most Clang 7 warnings (fixes #4972)

rtgui/batchqueue.h

@@ -99,6 +99,8 @@ private:
     bool saveBatchQueue ();
     void notifyListener ();
 
+    using ThumbBrowserBase::redrawNeeded;
+
     BatchQueueEntry* processing;  // holds the currently processed image
     FileCatalog* fileCatalog;
     int sequence; // holds the current sequence index

rtgui/filebrowser.h

@@ -61,6 +61,8 @@ class FileBrowser  : public ThumbBrowserBase,
 private:
     typedef sigc::signal<void> type_trash_changed;
 
+    using ThumbBrowserBase::redrawNeeded;
+
     IdleRegister idle_register;
     unsigned int session_id_;
 

rtgui/guiutils.h

@@ -194,6 +194,8 @@ private:
     ExpanderBox* expBox;        /// Frame that includes the child and control its visibility
     Gtk::EventBox *imageEvBox;  /// Enable/Disable or Open/Close arrow event box
 
+    using Gtk::Container::add;
+
     /// Triggered on opened/closed event
     bool on_toggle(GdkEventButton* event);
     /// Triggered on enabled/disabled change -> will emit a toggle event to the connected objects