Add support for stretchable spacers to XRC wxToolBar handler.

Add support for stretchable spacers to XRC wxToolBar handler.

Notice that currently only stretchable spacers are supported via
separator-like "space" XRC element. If we ever add support for fixed spacers
in the toolbar we should do it via its fixed sub-element, e.g. they would be
specified with <fixed>1</fixed> in XRC file.

Also use spacers instead of separator in the XRC sample and ensure that the
toolbars in it are resized.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63172 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775