Fix tab navigation bug with radio boxes without enabled items.

Fix tab navigation bug with radio boxes without enabled items.

There was a bug similar to the one in the preceding commit with radio boxes
under wxMSW too: if all radio box buttons were disabled (or hidden, although
this should be much more rare in practice, unlike the disabled case which was
deemed to be rare in r74583 commit message but turned out to actually happen),
the radio box still pretended to accept focus but didn't really do it.

Fix this by allowing to override wxWindow::CanBeFocused() and do it in
wxRadioBox to check whether we have any enabled visible items.

Also add a check for CanBeFocused() to wxControlContainer code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74586 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775