Alpha8: More Grid improvements

New Features

• ComboBox version updated to 3.0.1
• vaadin-upload is added to webjars bom #2804
• ComponentRenderer for Grid added #2720
• Added API for freezing columns in the Grid #2784
• Added API for rendering item details for Grid rows #2772
• Flow servlet and other Spring Servlets work better together now #2660
• Flow Spring tutorial app is added: https://github.com/vaadin/flow-spring-tutorial

All Changes

https://github.com/vaadin/flow/milestone/57?closed=1