Add variant dialogs trigger re-processing of imagesets too early

The UI for editing image set definitions has changed (ESSENTIALS-821). There are two dialogs involved now for creating a new image variant. The first asks for the name of the variant.

When clicking OK the node is saved and the image updater script is run. This is too soon, because the configuration details for the variant still need to be specified: that is done in the second dialog. The log of the script of the first run contains all kind of error messages, which is logical because the data it needs is not yet there. But it may confuse developers:

No width and height available for image variant myhippoproject:testthree. Skipping variant for node /content/gallery/myhippoproject/samples/viognier-grapes-188185_640.jpg/viognier-grapes-188185_640.jpg

Only when the specification details are saved the updater script should be run.