fix #11518: update Insight UI context menu tests (rebased)

Insight's UI changed and the context menu tests had to change accordingly. This required extension of functionality so that the relevant keywords could work down into submenus.

This PR fixes https://trac.openmicroscopy.org.uk/ome/ticket/11518 and rebases https://github.com/openmicroscopy/openmicroscopy/pull/1606 to make `./build.py -f components/tests/ui/build.xml ui-test-insight -DTEST=menus` pass after https://github.com/openmicroscopy/openmicroscopy/pull/1503 is merged into an up-to-date `develop`.

In general, the tests seem to pass far more reliably locally than on Hudson.