change _build* to rather use stateless widgets

[rickspencer3]

.//lib/src/document_form.dart: List _buildFormFields(BuildContext context) {
.//lib/src/document_list_view.dart: List _buildTitleRowChildren(Document doc) {
.//lib/src/document_list_view.dart: Widget _buildSubtitle(Document doc) {

Using _build* functions in this matter is considered an anti-pattern. It is just as easy to create stateless widgets. The idea is that the stateless widgets will just be moved around in the widget tree, and won;t be rebuilt unless needed.