Reorder list of files

Cleanup.
Rename NotificationItem.qml to NotificationBubble.qml

It's much more appropriate.
Create notifications daemon from the main thread

This should resolve some issues.
Make QML engine accessible from other classes

Implement a method to return the QML engine, which might be used
elsewhere without the need for a ShellUi.
Import QtQuick.Layouts from NotificationBubbleStyle

Fix missing import.
Make notification properties accessible to styles

Styles now can access notification properties such as summary, body and
icon.

Issue: #20
Don't use FluidUi.Icon for notifications

Just use a plain Image item so we can load icons from URLs.

Issue: #20
Fix notification bubble size

Correct style sizing.

Issue: #20
Fix icon loading in notification bubble style

Icon component size is 0x0 if it's not visible.

Issue: #20
Drop shadow and slight gradient for notification bubbles

Improve the default style.

Issue: #20