fix #11368: adjust event dispatch in Insight agents (rebased onto develop)



This is the same as gh-1480 but rebased onto develop.

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The crash on deleting ROIs then saving was intermittent, so do a bunch of them to make sure that Insight doesn't crash and that they really are deleted and don't come back from the server.

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/11368.