Script Editor: use the framework to choose files

@dscho, I wanted to get your feedback on this. This updates the Script Editor to use the UIService's `chooseFile` method rather than hardcoding `JFileChooser`. The advantage is that in legacy mode, it will reuse ImageJ1's file chooser, including respecting the "Use JFileChooser for Open/Save" setting.

However, the problems are:

* There is no API to override the file chooser's title; I added this on SciJava Common's [file-chooser-titles branch](https://github.com/scijava/scijava-common/tree/file-chooser-titles) but it is a breaking change which will need to be part of the `3.0.0` release.
* There is no API for specifying file filters. We could recapitulate an agnostic filtering system, but it would be more work. Do you think the Script Editor suffers from not being able to filter by, say, JAR extension in the relevant cases?