Generalize the File Open command

It should not limited to images, but rather work with any input file.
The current implementation should be extracted to a service (perhaps
part of IOService), but works for the moment to add support for text.
DefaultTextService: always wrap in "<html><body>"

This is necessary (at least, the "<html>" prefix is) for Java's HTML
rendering using JEditorPane.
SwingTextDisplayPanel: use a JEditorPane

Unlike with JTextArea, this allows for the rendering of HTML.