It's part of #331.

With gtk3 the samples app crashes with the following error, when the ComboBox sample is opened:
```
Gdk:ERROR:/build/buildd/gtk+3.0-3.12.2/./gdk/gdkcairo.c:193:gdk_cairo_surface_paint_pixbuf: assertion failed: (cairo_image_surface_get_format (surface) == CAIRO_FORMAT_RGB24 || cairo_image_surface_get_format (surface) == CAIRO_FORMAT_ARGB32)
Stacktrace:

  at <unknown> <0xffffffff>
  at (wrapper managed-to-native) Gdk.CairoHelper.gdk_cairo_set_source_pixbuf (intptr,intptr,double,double) <IL 0x00012, 0xffffffff>
  at Gdk.CairoHelper.SetSourcePixbuf (Cairo.Context,Gdk.Pixbuf,double,double) <IL 0x0002f, 0x00073>
  at Xwt.GtkBackend.GtkImage.DrawPixbuf (Cairo.Context,Gdk.Pixbuf,double,double,Xwt.Backends.ImageDescription) [0x00058] in /home/sevo/Dokumente/Projects/xwt-sevoku.git.fix/Xwt.Gtk/Xwt.GtkBackend/ImageHandler.cs:405
  at Xwt.GtkBackend.GtkImage.Draw (Xwt.Backends.ApplicationContext,Cairo.Context,double,double,double,Xwt.Backends.ImageDescription) [0x000fb] in /home/sevo/Dokumente/Projects/xwt-sevoku.git.fix/Xwt.Gtk/Xwt.GtkBackend/ImageHandler.cs:382
  at Xwt.GtkBackend.ImageRenderer.OnRender (Cairo.Context,Gtk.Widget,Gdk.Rectangle,Gdk.Rectangle,Gtk.CellRendererState) [0x0004b] in /home/sevo/Dokumente/Projects/xwt-sevoku.git.fix/Xwt.Gtk/Xwt.GtkBackend.CellViews/CustomCellRendererImage.cs:79
  at Gtk.CellRenderer.Render_cb (intptr,intptr,intptr,intptr,intptr,int) <IL 0x00053, 0x001f0>
  at (wrapper native-to-managed) Gtk.CellRenderer.Render_cb (intptr,intptr,intptr,intptr,intptr,int) <IL 0x00028, 0xffffffff>
  at <unknown> <0xffffffff>
  at (wrapper managed-to-native) object.wrapper_native_0x7fd3843deaa0 (intptr,intptr) <IL 0x00026, 0xffffffff>
  at (wrapper native-to-managed) object.wrapper_native_0x7fd3843deaa0 (intptr,intptr) <IL 0x00024, 0xffffffff>
  at <unknown> <0xffffffff>
  at (wrapper managed-to-native) object.wrapper_native_0x7fd3843deaa0 (intptr,intptr) <IL 0x00026, 0xffffffff>
  at (wrapper native-to-managed) object.wrapper_native_0x7fd3843deaa0 (intptr,intptr) <IL 0x00024, 0xffffffff>
  at <unknown> <0xffffffff>
  at (wrapper managed-to-native) Gtk.Application.gtk_main () <IL 0x0000e, 0xffffffff>
  at Gtk.Application.Run () <IL 0x00001, 0x0000b>
  at Xwt.GtkBackend.GtkEngine.RunApplication () [0x00000] in /home/sevo/Dokumente/Projects/xwt-sevoku.git.fix/Xwt.Gtk/Xwt.GtkBackend/GtkEngine.cs:156
  at Xwt.Application.<Run>m__0 () [0x00006] in /home/sevo/Dokumente/Projects/xwt-sevoku.git.fix/Xwt/Xwt/Application.cs:97
  at Xwt.Toolkit.InvokePlatformCode (System.Action) [0x00018] in /home/sevo/Dokumente/Projects/xwt-sevoku.git.fix/Xwt/Xwt/Toolkit.cs:280
  at Xwt.Application.Run () [0x00023] in /home/sevo/Dokumente/Projects/xwt-sevoku.git.fix/Xwt/Xwt/Application.cs:96
  at Samples.App.Run (Xwt.ToolkitType) [0x00013] in /home/sevo/Dokumente/Projects/xwt-sevoku.git.fix/TestApps/Samples/App.cs:17
  at GtkTest.MainClass.Main (string[]) [0x00002] in /home/sevo/Dokumente/Projects/xwt-sevoku.git.fix/TestApps/Gtk3Test/Main.cs:36
  at (wrapper runtime-invoke) <Module>.runtime_invoke_void_object (object,intptr,intptr,intptr) <IL 0x00050, 0xffffffff>
```

When using Gtk2, the images are sized automatically to 16x16 (by gtk itself?), if the size is set to Size.Zero. And I think its logical to set some real size as default instead of zero. So this patch sets the same default size which is used in gtk2, but already in the xwt code, fixing the issue with gtk3.