Convert search to use a GtkFileSystemModel

Convert search to use a GtkFileSystemModel

Replace the list model code with the file system model and use all the
file system model API niceties we get from that.
Also adds the function _gtk_file_system_model_add_and_query_file() which
g_file_query_info()'s the file before adding it, so it gets added with
the right information.