Set all attached section properties before emitting change signals.

If we have bindings to the section properties, e.g. implementing manual
section header creation, we want previousSection, section and
nextSection to be set before emitting the change signals to prevent
different results each time the binding is run.

Change-Id: Id3a0b4a53419681f35102c9e7c620b5c6112ebb0
Reviewed-by: Martin Jones <martin.jones@jollamobile.com>