Implement wxGetActiveWindow() for wxX11.

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