Add support for wxAuiManager and wxAuiPaneInfo to XRC

Add XRC handler for wxAuiManager and include the existing wxAuiNotebook
handler into it (but notice that wxAuiToolBar handler added by a later #15686
in f269f868d7 remains separate).

Also update the AUI dialog in the sample and stop hardcoding its size in
pixels.

See #13520.

samples/xrc/xrcdemo.cpp

@@ -42,7 +42,7 @@
 #endif // wxUSE_RIBBON
 
 #if wxUSE_AUI
-    #include "wx/xrc/xh_auinotbk.h"
+    #include "wx/xrc/xh_aui.h"
     #include "wx/xrc/xh_auitoolb.h"
 #endif // wxUSE_AUI
 
@@ -91,7 +91,7 @@ bool MyApp::OnInit()
 #endif
 
 #if wxUSE_AUI
-    wxXmlResource::Get()->AddHandler(new wxAuiNotebookXmlHandler);
+    wxXmlResource::Get()->AddHandler(new wxAuiXmlHandler);
     wxXmlResource::Get()->AddHandler(new wxAuiToolBarXmlHandler);
 #endif