Fix and enhance support for client data in wxRibbonButtonBar.

Fix and enhance support for client data in wxRibbonButtonBar.

Add the possibility to retrieve the client data associated with a button and
not only set it (which wasn't very useful on its own).

Also allow having both typed (owned) and untyped (not owned) client data, as
in the other wxWidgets controls.

To avoid confusion between two different kinds of data, remove "client_data"
argument from the functions adding buttons and provide separate methods with
distinct names for setting and getting client data.

Closes #14630.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72529 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775