Port to USDPL

[NGnius]

Describe what you'd like to be able to do
Rewrite PowerTools to work with USDPL. This would involve a complete rewrite of the back-end and some non-invasive changes to the front-end.

Describe alternatives you've considered
Status quo (staying with Decky's built-in back-end API) is really the only realistic alternative, but it does not meet my requirements for maintainability, portability, and security.

Additional context
USDPL: https://github.com/NGnius/usdpl-rs

This would also be a good time to shake off some minor technical debt and expand the tools at everyone's disposal.

[NGnius]

This is mostly complete. The UI is the last remaining piece (and therefore also integratiom testing) to do, but that cannot be completed while I'm away from my main dev PC.