ChannelsTool: add shift+control+Z shortcut

It seems ImageJ1 acquired this shortcut combination recently,
so we do the same for ImageJ2.
Guard against null services in IOPlugins

It can be the case that an ImageJ context is initialized with an
IOService, but not e.g. a DatasetService or TextService or LUTService.
In those cases, the corresponding IOPlugin needs to fail gracefully
rather than throwing an NPE.