Implement wxRB_SINGLE support in wxGTK

Create hidden radio button in wxGTK when using wxRB_SINGLE, as
this lets us deactivate the shown button by activating the hidden one.
GTK does not allow deactivating radio buttons by calling them with
gtk_toggle_button_set_active( ..., FALSE), so we need to work around
it like this.

Also update the documentation to mention that this works in wxGTK now.

See #17022.

Closes #23652.

Closes #23677.

include/wx/gtk/radiobut.h

@@ -55,6 +55,8 @@ protected:
 
 private:
     typedef wxControl base_type;
+    // Only used by wxRB_SINGLE
+    GtkWidget* m_hiddenButton = nullptr;
 
     wxDECLARE_DYNAMIC_CLASS(wxRadioButton);
 };

interface/wx/radiobut.h

@@ -33,9 +33,10 @@
            When this style is used, no other radio buttons will be turned off
            automatically when this button is turned on and such behaviour will
            need to be implemented manually, in the event handler for this
-           button. This style is currently only supported in wxMSW, in the
-           other ports it can be specified, but single radio buttons can't be
-           turned off, making them not very useful.
+           button. This style is currently only supported in wxMSW and wxGTK
+           (since version 3.3.0). In the other ports it can be specified, but
+           single radio buttons can't be turned off, making them not very
+           useful.
     @endStyleTable
 
     @beginEventEmissionTable{wxCommandEvent}

src/gtk/radiobut.cpp

@@ -92,7 +92,22 @@ bool wxRadioButton::Create( wxWindow *parent,
         }
     }
 
+    // GTK does not allow a radio button to be inactive if it is the only radio
+    // button in its group, so we need to work around this by creating a second
+    // hidden radio button.
+    if (HasFlag(wxRB_SINGLE))
+    {
+        m_hiddenButton = gtk_radio_button_new( nullptr );
+        m_widget = gtk_radio_button_new_with_label_from_widget( GTK_RADIO_BUTTON(m_hiddenButton), label.utf8_str() );
+        // Since this is the second button in the group, we need to ensure it
+        // is active by default and not the first hidden one.
+        gtk_toggle_button_set_active( GTK_TOGGLE_BUTTON(m_widget), TRUE );
+    }
+    else
+    {
         m_widget = gtk_radio_button_new_with_label( radioButtonGroup, label.utf8_str() );
+    }
+
     g_object_ref(m_widget);
 
     SetLabel(label);
@@ -133,9 +148,12 @@ void wxRadioButton::SetValue( bool val )
     }
     else
     {
-        // should give an assert
-        // RL - No it shouldn't.  A wxGenericValidator might try to set it
-        //      as FALSE.  Failing silently is probably TRTTD here.
+        // Normal radio buttons can only be turned off by turning on another
+        // button in the same group, but the single ones can be turned off
+        // manually, which is implemented by turning a hidden button on, as
+        // it's the only way to do it with GTK.
+        if (HasFlag(wxRB_SINGLE))
+            gtk_toggle_button_set_active( GTK_TOGGLE_BUTTON(m_hiddenButton), TRUE );
     }
 
     g_signal_handlers_unblock_by_func(