Use Qml ApplicationWindow instead of a QQuickView.

- starts the application directly from qml/main.qml without
  initializing anything else. Qml manages the application
  window (including title, menubar, etc).
- change the color of the current section (#f2f2f2). later,
  this color should be fetched from the cover's most dominant color.
- round up the section image's corners.
- implement setData() in CollectionModel.
- make CollectionModel's roles names private.