Implement wxGetActiveWindow() for wxX11.

Now that wxGetActiveWindow() is used when showing modal dialogs, it became
critical to implement it as otherwise an assert happens every time a dialog is
shown and because creating the assert dialog itself results in another (same)
assert, this immediately results in a crash.

So implement it even if in a very trivial (but hopefully not too incorrect)
way.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62003 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/x11/window.cpp

@@ -1641,9 +1641,7 @@ bool wxWindowX11::SetForegroundColour(const wxColour& col)
 
 wxWindow *wxGetActiveWindow()
 {
-    // TODO
-    wxFAIL_MSG(wxT("Not implemented"));
-    return NULL;
+    return wxGetTopLevelParent(wxWindow::FindFocus());
 }
 
 /* static */