libhawaiishell: Add missing title property to ElementItem

Forgot to add the Q_PROPERTY.
libhawaiishell: Add label property to ElementItem

This will be useful to indicators implemented as elements.
The status menu will use this property along with the iconName property
to show the element in a summary.
libhawaiishell: Containment attached object

Just like we did with elements.
The containment is exposed with an attached object.

Issue: #150
libhawaiishell: Add ElementItem::setContextProperty documentation
client: Create ContainmentItem

Create a ContainmentItem object to make the attached object available.

Issue: #150
core: Add ContainmentItem

Without this, containments can't access the Containment attached
object.

Issue: #150
containments-panel: Use Containment attached object

Issue: #150
libhawaiishell: Move immutable property from QuickView to Containment

The configuration logic does really belong here.

Issue: #151
libhawaiishell: Move configuring property from QuickView to Containment

The configuration logic does really belong here.

Issue: #151
client: Remove useless context properties on elements

Both Ui and panel are not relevant anymore.
libhawaiishell: Move element handling to Containment and ContainmentItem

Elements are added to containments, the Containment attached object
creates elements graphical representation.

Issue: #151
client: Add elements to containments

This is much more clean.

Issue: #151
containments-panel: Get rid of panel context property

Use the Containment attached object everywhere.

Issue: #150
Issue: #151
libhawaiishell: Expose element and containment root object

It's worth having a method that returns the actual root object
so that it can be accessed elsewhere.

Issue: #150
Remove leftovers

Issue: #148