Don't consider removed items in print composer z-list

Currently, when items are removed from the print composer, they are not removed from the z-list. This causes the raise / lower commands to apparently have no effect, if the next/previous element in the z-list are items which were removed from the composition.

This patch modifies `raiseItem` and `lowerItem` to ignore items which are not part of the composition when searching for the new z position.

Funded by Sourcepole QGIS Enterprise.