EntityIdInput expert keeps track of raw value when set to deleted entity

When the value of the expert is set to an ID of an entity which is deleted, then the input field is
empty. Because of this, when asking for the value (via rawValue()), the expert did return null
instead of the ID of the deleted/missing entity.
This was wrong behavior since displaying an empty string for missing entities is just an decision
of how to visualize the value. rawValue() should still return the same value as set before. Only
after the user changes the input's value, the value should be set to null (empty) or another value.

One case not clearly defined still is what happens when the value is set to a missing entity, so
an empty string will be displayed in the box, but then the user types something, then removes the
typed string again so the text in the box is an empty string again. Should rawValue() could now
either return null (current behavior) since the user changed the value in the meanwhile, or it
could be reset to the missing entity ID which was set before.

Effect in the UI: When clicking edit on a Snak using a deleted item, then pressing cancel, the view
                  did not display "<id> (Deleted item)" anymore as it did before clicking edit.
                  This change fixes this, so the label will be displayed correctly after cancel.

Change-Id: I678b7b2724857c2a950c06e3d9ec72afb79170e9
Merge "EntityIdInput expert keeps track of raw value when set to deleted entity"