search: Minor search-location-dialog refactoring

Merged Iain Lane requested to merge wip/laney/search-always-set-dialog-member into master

search: Factor out creation of Place objects.

If we always construct these objects via a constructor, then we can help to avoid bugs like that fixed by 2fb0ff25.

Also stop using the slice allocator in favour of g_new0.

Edited by Iain Lane

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