fix #11090: disable Cancel All button when not needed

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/11090 -- try starting imports and cancelling some and make sure that the Cancel All button is only enabled when it would actually do something, and make sure that it still actually works when you click it.

(The `CancelAction` is used under the hood for all the Cancel All buttons to keep their enabledness up to date.)