Add wxRibbonBar::{Show,Hide}Panels() and ArePanelsShown() accessor.
Also add a toggle button to the sample to test the new functionality
(unfortunately it couldn't be done by a control in the ribbon itself as there
would be no way to show the panels back then).
Closes#12707.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66612 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775