Cleanup TableModelDialog and migrate away from deprecated JApplet

This adapts the model dialog for the JTable to make sure all
modifications to the Swing widget are done in the AWT event dispatcher
thread and changes the preview to use the `EmbeddedSwingComposite`,
which creates an `SWT_AWT` backed by a `JRootPane`, rather than a
`JApplet`.

The `JApplet` class has been removed with Java 26, so this migration is
strictly necessary for WindowBuilder to remain compatible with upcoming
Java releases.

Author: ptziegler