don't check whether the window is shown and enabled in AcceptsFocus() itself

don't check whether the window is shown and enabled in AcceptsFocus() itself
as it makes overriding it in derived classes problematic; provide a separate
non virtual CanAcceptFocus() method checking whether the window accepts focus
and if it can accept it now and use it instead of AcceptsFocus(); documented
AcceptsFocus() and AcceptsFocusFromKeyboard()

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45055 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775