Fix tab navigation bug with static boxes without enabled children.

wxControlContainer::AcceptsFocusFromKeyboard() returned true even if the
control didn't have any currently enabled -- and hence accepting focus --
children. This resulted in strange wxEVT_NAVIGATION_KEY propagation as it
unexpectedly wasn't handled in the control which pretended to accept focus and
instead bubbled up back into the parent, resulting in the focus returning to
the first child of the parent instead of skipping the static box with disabled
children and going to the next enabled child.

Fix this by checking that we have children that can be focused right now and
not only children that are focusable. Notice that this doesn't take care of
calling wxWindow::SetCanFocus() correctly when the children enabled/disabled
state changes so there might still be other problems, notably under wxGTK
where SetCanFocus() does something non-trivial, but it at least improves
things under wxMSW.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74585 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
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
Document wxKill(wxSIGTERM) reliance on having an open window in wxMSW.

wxKill(wxSIGTERM) doesn't work if the child process has no open windows as it
doesn't have anything to post the WM_QUIT message to.

Document this and point out that wxKill(wxSIGKILL) still works in this case.

Closes #15356.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74587 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775