WIP: Script Editor: use the framework to choose files

This calls the UIService's chooseFile method, rather than hardcoding
usage of JFileChooser. This is especially important for scenarios like
the LegacyUI being visible, since in many ways it reuses the SwingUI,
but with many functions implemented differently, including chooseFile.

The reason this is a "WIP" is because it does not yet convert _all_ uses
of JFileChooser in the TextEditor to use the UIService instead. It
cannot fully do so, because the TextEditor relies on being able to
control more details of the JFileChooser such as its filter. Before this
branch can be completed, we will need to enhance the UIService and
UserInterface APIs to be powerful enough to do that in an agnostic way.