Add support for fixed spacers and labels to wxAuiToolBar XRC handler.

Allow specifying "width" and "proportion" attributes for the "space" elements
and add "label" element support.

Closes #15964.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75920 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Don't delete child controls when deleting wxStaticBoxSizer.

This is an incompatible change compared to 2.8 which can make the existing
code crash and it also goes against the usual rule that the windows are never
owned by sizers, only other windows.

Closes #15698.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75921 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775