Remove deprecated GTK3 code, fixes #6103 (#6113)

Replaces the following deprecated Gtk classes throughout the codebase by their non-deprecated counterparts. Style, HBox, VBox, HPaned, VPaned, HScrollbar, VScrollbar, VSeparator, HSeparator, Stock, Table, VScale and HScale.

rtgui/guiutils.h

@@ -185,7 +185,7 @@ public:
  *
  * Warning: once you've instantiated this class with a text label or a widget label, you won't be able to revert to the other solution.
  */
-class MyExpander final : public Gtk::VBox
+class MyExpander final : public Gtk::Box
 {
 public:
     typedef sigc::signal<void> type_signal_enabled_toggled;
@@ -199,7 +199,7 @@ private:
     bool enabled;               /// Enabled feature (default to true)
     bool inconsistent;          /// True if the enabled button is inconsistent
     Gtk::EventBox *titleEvBox;  /// EventBox of the title, to get a connector from it
-    Gtk::HBox *headerHBox;
+    Gtk::Box *headerHBox;
     bool flushEvent;            /// Flag to control the weird event mechanism of Gtk (please prove me wrong!)
     ExpanderBox* expBox;        /// Frame that includes the child and control its visibility
     Gtk::EventBox *imageEvBox;  /// Enable/Disable or Open/Close arrow event box
@@ -373,13 +373,15 @@ public:
 };
 
 /**
- * @brief subclass of Gtk::HScale in order to handle the scrollwheel
+ * @brief subclass of Gtk::Scale in order to handle the scrollwheel
  */
-class MyHScale final : public Gtk::HScale
+class MyHScale final : public Gtk::Scale
 {
 
+protected:
     bool on_scroll_event (GdkEventScroll* event) override;
     bool on_key_press_event (GdkEventKey* event) override;
+
 };
 
 /**
@@ -391,7 +393,7 @@ private:
 
     Glib::ustring title_;
     Gtk::FileChooserAction action_;
-    Gtk::HBox box_;
+    Gtk::Box box_;
     Gtk::Label lbl_;
     std::string filename_;
     std::string current_folder_;
@@ -473,7 +475,7 @@ typedef enum RTNav {
 /**
  * @brief Handle the switch between text and image to be displayed in the HBox (to be used in a button/toolpanel)
  */
-class TextOrIcon final : public Gtk::HBox
+class TextOrIcon final : public Gtk::Box
 {
 
 public: