Add support for hiding sizer items in XRC

Handle the `<hideitems>` property for sizers and document it.

Also group `minsize` together with `hideitems` in `stdSizerProperties`, which
is used by all sizer classes except `wxStdDialogButtonSizer` in the XRC
schema.

docs/changes.txt

@@ -80,6 +80,7 @@ All (GUI):
 - Allow requesting modern (3.x+) OpenGL version in wxGLCanvas (Fabio Arnold).
 - Add wxActivityIndicator.
 - Add wxWindow::FromDIP() for simpler high DPI support.
+- Allow initially hiding sizer items in XRC (Bogdan Iordanescu).
 - Allow customizing window shown by wxBusyInfo.
 - Add wxAddRemoveCtrl.
 - Add wxAppProgressIndicator for MSW (Chaobin Zhang) and OS X (Tobias Taschner).