Added menu item for deleting CompoundDataPlugValueWidget children.

This allows items to be removed from CustomAttributes nodes and the like.

Fixes #1031.
Added SceneTestCase.assertBuiltInSetsComplete() method.

We were already checking that all objects referenced by sets do exist - this extends upon that by checking that all cameras, lights and coordinate systems that exist are referenced by the appropriate built in sets. The test has to be disabled for a couple of tests involving Alembic files, because we don't have a way of flagging cameras in those.
Added "isolate from" capability to GafferScene.Isolate node.

This causes the effects of isolation to be applied only below a certain location, making it easy to protect the rest of the scene from pruning.

Fixes #1057.
Merge pull request #1099 from johnhaddon/compoundDataPlugDeleteUI

Added menu item for deleting CompoundDataPlugValueWidget children.
Merge pull request #1100 from johnhaddon/isolateFrom

Added "isolate from" capability to GafferScene.Isolate node
Update Changes
Bumped version to 6.0.0
Added script for automating releases.
Updated IE public build script to use MILESTONE_VERSION.

Also added comment clarifying that we only use the script for building dependencies at present.
Renamed IE publicBuild script to publicDependenciesBuild.

Because that's what we use it for now.
Added script for automating public releases at IE.

This uses the standard release script for doing the work. We can't use that directly at IE because we need to bypass custom wrappers to get a vanilla build environment.