IGV: cannot edit forms with NetBeans GUI builder

After JDK-8264795, it is no longer possible to edit IGV forms using the NetBeans GUI builder.

HOW TO REPRODUCE:

1. Open the IdealGraphVisualizer project on NetBeans 12.3, selecting "Open Required Projects" as well.
2. In the "Projects" window, Open Settings -> Source Packages -> com.sun.hotspot.igv.settings -> ViewPanel.java
3. We expect NetBeans to open ViewPanel.java in "Design" mode (see expected.png) but it opens instead the file in "Source" mode (see actual.png), without the possibility of switching to "Design".

POSSIBLE SOLUTION:

Move all .form files created by NetBeans to the same directory as their corresponding .java files.