Set cwd to ArtDirectory before XRC Preview

This shouldn't matter for C++ which will be using embedded art, but for
XRC it increases the chance that it will load the requested art (which
doesn't have a path specifier).

src/previews.cpp

@@ -166,6 +166,9 @@ void Preview(Node* form_node)
             return;
         }
 
+        tt_cwd cwd(true);
+        wxSetWorkingDirectory(Project.ArtDirectory().make_wxString());
+
         XrcCompare dlg_compare;
         if (!dlg_compare.DoCreate(wxGetMainFrame(), form_node))
         {
@@ -256,6 +259,9 @@ void PreviewXrc(Node* form_node)
             return;
         }
 
+        tt_cwd cwd(true);
+        wxSetWorkingDirectory(Project.ArtDirectory().make_wxString());
+
         switch (form_node->getGenName())
         {
             case gen_wxDialog: