Reset pending focus when losing focus in wxGTK

It could happen that the window remained as the global "pending focus"
even after it lost its focus, resulting in FindFocus() still returning
it as the currently focused window, even though it didn't have focus any
more.

This notably broke focus logic of wxCompositeWindow and could result in
missing wxEVT_KILL_FOCUS for windows using it, such as wxDatePickerCtrl,
and quite likely was responsible for other focus problems in wxGTK as
well.

src/gtk/window.cpp

@@ -4374,7 +4374,13 @@ bool wxWindowGTK::GTKHandleFocusIn()
         gtk_im_context_focus_in(m_imContext);
 
     gs_currentFocus = this;
-    gs_pendingFocus = NULL;
+
+    if ( gs_pendingFocus )
+    {
+        wxLogTrace(TRACE_FOCUS, "Resetting pending focus %s on focus set",
+                   wxDumpWindow(gs_pendingFocus));
+        gs_pendingFocus = NULL;
+    }
 
 #if wxUSE_CARET
     // caret needs to be informed about focus change
@@ -4406,6 +4412,15 @@ bool wxWindowGTK::GTKHandleFocusOut()
     // handler issues a repaint
     const bool retval = m_wxwindow ? true : false;
 
+    // If this window is still the pending focus one, reset that pointer as
+    // we're not going to have focus any longer and DoFindFocus() must not
+    // return this window.
+    if ( gs_pendingFocus == this )
+    {
+        wxLogTrace(TRACE_FOCUS, "Resetting pending focus %s on focus loss",
+                   wxDumpWindow(this));
+        gs_pendingFocus = NULL;
+    }
 
     // NB: If a control is composed of several GtkWidgets and when focus
     //     changes from one of them to another within the same wxWindow, we get