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.
Updated Changes file.
Merge remote-tracking branch 'upstream/master' into gafferseed
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
Merge remote-tracking branch 'upstream/master' into gafferseed
preparing PR.