Separate Launcher window from contents

This will help us styling Launcher.
Style API

We add the Hawaii.Shell.Styles module to offer an API for QML styles.
It's inspired by the QtQuick Controls style API.

Issue: #15
Separate Panel window from contents.

This will make styling Panel easier.
Reference styled item

Might be useful for styles to know the styled item size and such.

Issue: #15
Automatically determine Panel size

PanelWindow determines its size automatically.
Default Panel style

We include a default Panel style.
Later we'll load styles from disk.