When searching snap targets in composer view, pick the closest one

There are some cases where items moved in the composer view do not snap to the expected guide. For instance, it is impossible to move and align the label "Demo Print Composer" to the left of the label "QGIS Enterprise 13.03" in this example: http://smani.fedorapeople.org/druckvorlage.qpt

This patch ensures the closest snap guide is picked.

Funded by Sourcepole QGIS Enterprise.