Fix a couple sample app bugs / double marker placement issues

[msmollin]

Overview

This fixes a couple issues that cropped up in the sample app under testing, adds a feature request, and fixes a bug with synchronous scene loading.

Proposed Changes

• Make the style sheet picker actually apply label density / detail level - Fixes #374
• Remove restoring markers from sync load methods and depend upon callback doing everything - Fixes #370
• Add route type selector: Fixes #317