app: Use QQmlApplicationEngine

Closes #20
Deferred preflet item creation

Create QML component as soon as the model is populated and create items
the first time a preflet is requested.

Closes #21
app: Simplify layout a little bit

Use ColumnLayout as a top level item.
app: Make it flickable

Turn the main page into a flickable item.

Closes #22
app: Remove categories model