Fix accelerators in appended submenu items in wxGTK.

They were not previously taken into account because we didn't call
gtk_widget_add_accelerator() on the right GtkAccelGroup.

Closes #16050.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76113 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775