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.

Source/Core/DolphinWX/Main.cpp

@@ -484,7 +484,7 @@ bool wxMsgAlert(const char* caption, const char* text, bool yes_no, int /*Style*
 	if (wxIsMainThread())
 #endif
 		return wxYES == wxMessageBox(StrToWxStr(text), StrToWxStr(caption),
-				(yes_no) ? wxYES_NO : wxOK, wxGetActiveWindow());
+				(yes_no) ? wxYES_NO : wxOK, wxWindow::FindFocus());
 #ifdef __WXGTK__
 	else
 	{