Add test for String conversion to images

[imagejan]

We should ensure that String values are converted to ImagePlus (or Dataset) objects only if there actually is an already opened image matching the conversion request.

If there is no matching image, we shouldn't try to convert (and maybe return null), so that other converters can take over and load e.g. from a file path.

[imagejan]

What I did so far in this pull request:

• https://github.com/imagej/imagej-legacy/commit/b259c9ab4d19fcdeb5a9c584f3bc0ce67275db29 adds a test to ensure that non-existing titles or image IDs are not picked up by StringToImagePlusConverter. This test fails with the current state of master.
• https://github.com/imagej/imagej-legacy/commit/796c9d2649a806fcb0c3bd26eeb26dcddb978206 adds an overridden canConvert(Object, Type) in addition to canConvert(Object, Class) which makes the above test pass. Actually, the Class-based signature doesn't seem to get called at all under test conditions, as the test also passes if I remove canConvert(Object, Class).
• https://github.com/imagej/imagej-legacy/commit/2dc041fdc794426086bdba98030344698d472b71 and https://github.com/imagej/imagej-legacy/commit/92ba2882cecb021ea0bd63b024d5d92f461f1b61 add additional tests for (OpenedImageTitle)String => Dataset conversion as well as for input parameters filled by String values in IJ.run option strings. I would have expected both tests to fail in absence of an explicit StringToDatasetConverter that takes care of providing active/opened images, but to my surprise, both tests actually pass.

Regarding the last point, I checked which class is returned when calling convertService.getHandler(nonExistentTitle, Dataset.class), and (unsurprisingly) we get io.scif.convert.StringToDatasetConverter (see also https://github.com/scifio/scifio/pull/443). But I was astonished to see that despite the SCIFIO converter kicking in here, the correct opened image seems to be provided nevertheless.

See also https://github.com/imagej/imagej-legacy/issues/246 for related discussion.

[maarzt]

This PR no longer works, due to changes in SCIFIO. There's also still a bug in StringToImagePlusConverter.convert(...) method. It doesn't check the destination type to be ImagePlus.

Closed in favor of #272

(@imagejan Sorry! I create a nearly identical PR before finding this one. But thanks to your PR, I found the but in the StringToImagePlusConverter, which would have otherwise caused some trouble.)