fix: clean up imports for 0.4.x
LegacyUI: implement choose file

Added an implementation for the chooseFile method. This is almost
identical to that of the SwingUI, and should potentially be consolidated
(except that we want to use the ImageJ application frame).
LegacyImageDisplayViewer: set active display

Since we aren't using the framework for actually displaying our legacy
images (they are rendered via ImageJ instead), we need to manually
inform the IJ2 framework what the active display is so it can be queried
by interested parties.
Add LegacyDisplayViewer interface

This is a marker interface. As both the ImagePlus and LegacyImage
display viewers do not use the framework for the actual image rendering
(and thus do not have a display window) they are treated specially by
the LegacyUI. A common interface simplifies the test for these display
viewers.