Avoid some GTK+ run-time errors when using wx{File,Dir}PickerCtrl

The wxGTK implementation uses wx{File,Dir}Button, which derives from
wxButton, but is not a GtkButton, so many operations which try to modify
it, such as changing it's style, are likely to result in errors.
See #17984

include/wx/gtk/filepicker.h

@@ -101,6 +101,8 @@ public:     // overrides
 protected:
     wxDialog *m_dialog;
 
+    virtual void DoApplyWidgetStyle(GtkRcStyle*) wxOVERRIDE;
+
 private:
     // common part of all ctors
     void Init() { m_dialog = NULL; }
@@ -170,6 +172,8 @@ public:     // overrides
 protected:
     wxDialog *m_dialog;
 
+    virtual void DoApplyWidgetStyle(GtkRcStyle*) wxOVERRIDE;
+
 public:    // used by the GTK callback only
 
     bool m_bIgnoreNextChange;

src/gtk/filepicker.cpp

@@ -158,6 +158,9 @@ void wxFileButton::SetInitialDirectory(const wxString& dir)
         wxGenericFileButton::SetInitialDirectory(dir);
 }
 
+void wxFileButton::DoApplyWidgetStyle(GtkRcStyle*)
+{
+}
 #endif // wxUSE_FILEPICKERCTRL
 
 #if wxUSE_DIRPICKERCTRL
@@ -331,4 +334,7 @@ void wxDirButton::SetInitialDirectory(const wxString& dir)
         wxGenericDirButton::SetInitialDirectory(dir);
 }
 
+void wxDirButton::DoApplyWidgetStyle(GtkRcStyle*)
+{
+}
 #endif // wxUSE_DIRPICKERCTRL