filechooserbutton: Simplify the code to preserve the selection while the dialog is...

filechooserbutton: Simplify the code to preserve the selection while the dialog is running

It used to fetch a possibly multiple selection from the GtkFileChooserDialog, and then
pick just the first item from the selection list.  But since GtkFileChooserButton
operates in single-selection mode only, it can simply use gtk_file_chooser_get_file()
instead.

Also, the right way to reset the selection for GTK_FILE_CHOOSER_ACTION_SELECT_FOLDER
is with gtk_file_chooser_select_file(), not with _set_current_folder_file().

Signed-off-by: Federico Mena Quintero <federico@gnome.org>