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/toolpanel.h

@@ -49,7 +49,7 @@ public:
 
 /// @brief This class control the space around the group of tools inside a tab, as well as the space separating each tool. */
 class ToolVBox :
-    public Gtk::VBox
+    public Gtk::Box
 {
 public:
     ToolVBox();
@@ -57,7 +57,7 @@ public:
 
 /// @brief This class control the space around a tool's block of parameter. */
 class ToolParamBlock :
-    public Gtk::VBox
+    public Gtk::Box
 {
 public:
     ToolParamBlock();