Fix focus detection on OS X by replacing wxGetActiveWindow with wxWindow...

...::FindFocus.

wxGetActiveWindow is implemented as "return NULL" on OS X, while
wxWindow::FindFocus works.  On Windows, the difference is in the use of
GetActiveWindow() vs. GetForegroundWindow().  A MSDN comment says:

> A system has only one active window, which GetForegroundWindow()
> returns. GetActiveWindow() seems to return the same window as
> GetForegroundWindow() if the foreground window belongs to the current
> thread. Otherwise, it always returns null, rather than the topmost
> window of the calling thread.

Since we are on the GUI thread, it shouldn't make any difference.