client: Indicator now contains the item for StatusMenu

Indicators' contents are now reparented inside this container and thus
can now be displayed inside StatusMenu.

Issue: #90
client: Add volume indicator

The first real indicator.

Issue: #90
client: Create StatusMenu from StatusArea

This way we can share the list of indicators and have StatusArea show
only the icon while StatusMenu reparent the items inside itself.

Issue: #90