Interpret default file name relatively to initial dir in wxGTK wxFileDialog.

Interpret default file name relatively to initial dir in wxGTK wxFileDialog.

Use the initial dialog directory to build the absolute path required by GTK+
in wxFileDialog instead of the current working directory. This makes more
sense and makes specifying the default directory and (just) the default file
name work correctly and avoids GTK+ warnings like

Gtk-CRITICAL **: gtk_file_folder_unix_get_info: assertion `strcmp (dirname, folder_unix->filename) == 0' failed

(and in some older versions of GTK+ -- 2.10? -- this even results in a crash).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70731 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775