19.5 Port for UE5.1

[CakeQ]

Hi. As I wanted to use Houdini for a project using the latest stream of UE5 (from the UE5-main branch on GitHub), I decided to resolve the compilation issues present within the 19.5 branch to support UE5.1

The port was broken into 3 parts:

• PhysX is gone, so have a few PhysX/Chaos ifdefs. Any plugins that use them just need the ifchecks removed and defaulted to whatever was within the Chaos engine block
• They removed the Editor style class. Switch to using FAppStyle
• UMaterial got refactored a bunch, and a lot of variables either got removed or made editor-only. In this case, everything Houdini needs is still accessible through a GetEditorOnlyData() function.

As I can see that the individual branches on your repository are for separate releases, I would assume you would want to create a 19.5-Unreal5.1 branch or something similar, but I don't know how to create a pull request without targeting an existing branch. If you were to create a new branch I would be able to change the target branch on this pull request.

If you have any questions or change requests please let me know!