Don't crash in wxWindowMac::OSXSimulateFocusEvents().

The code didn't properly account for the possibility of wxEVT_KILL_FOCUS
handler deleting the focused window (as happens e.g. in wxListCtrl inline edit
controls). E.g. rapidly clicking wxEditableListBox buttons could trigger a
crash here when sending wxEVT_SET_FOCUS to an already-deleted window.

Use wxWeakRef<> to check if the window is still alive.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75198 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775