Allow more flexible image input options

Currently, module inputs of type `Img` (and `ImagePlus`, `Dataset`, etc. via converters) are handled by [`ObjectWidget`](https://github.com/scijava/scijava-common/blob/master/src/main/java/org/scijava/widget/ObjectWidget.java) and [`SwingObjectWidget`](https://github.com/scijava/scijava-ui-swing/blob/master/src/main/java/org/scijava/ui/swing/widget/SwingObjectWidget.java) that offer a simple `JComboBox` with a list of convertible objects.
It would be nice to offer more flexibility via a `style` attribute, to allow:
* reading images from files (`style="file"`)
* choosing from either a list of open images, or a file (`style="file-or-image"`)

Another (maybe more elegant) option would be to create a singleton object that shows up as "Load from file..." option in any dropdown list for image inputs, thereby allowing to choose an open image or load from file at will.

See https://github.com/saalfeldlab/bigwarp/issues/42#issuecomment-340198033 for the discussion that triggered this issue.

Start by comparing ObjectWidget.java and SwingObjectWidget.java, then review the linked BigWarp discussion to resolve whether styles or a "Load from file..." option is preferred. Done means image inputs can select open images, files, or both according to the agreed design.

Written by the indexing model from the issue text.