LegacyUI: run file choosing on the EDT

Apparently, when IJ 1.x is the application frame, using a JFileChooser
can cause deadlocks if not run on the EDT. Observed in the
ij.io.OpenDialog class.

So, for the LegacyUI, since we're using a JFileChooser with an ImageJ as
our application frame, we should probably ensure it's run on the EDT.