Move GTK3 wxNO_BORDER handling to wxControl

So wxNO_BORDER works with other controls, such as wxBitmapButton.
Also use GTK prefix on ApplyCssStyle(), and add an overload that
creates the GtkCssProvider.

src/gtk/control.cpp

@@ -97,6 +97,11 @@ void wxControl::PostCreation(const wxSize& size)
 {
     wxWindow::PostCreation();
 
+#ifdef __WXGTK3__
+    if (HasFlag(wxNO_BORDER))
+        GTKApplyCssStyle("*{ border:none; border-radius:0; padding:0 }");
+#endif
+
 #ifndef __WXGTK3__
     // NB: GetBestSize needs to know the style, otherwise it will assume
     //     default font and if the user uses a different font, determined