[Feat]: Replace CustomTkInter with PyQt6

[Arcitec]

• CustomTkInter is a buggy library by one solo dude who quit development in June 2023. It has major issues with UI scaling and extremely slow GUI rendering, lots of bugs and rendering glitches.
• We have been talking about the need for a native rendering library that's faster and better.
• PyQt6 has a very similar API and event loop, allowing us to keep our native rendering (no web UI, meaning no browser, meaning no GPU VRAM being wasted by the UI, and no complicated server/client architecture).
• PyQt6 is a professional GUI library operated by a company that has existed for over 30 years and is well-funded. It's free for GPL-based software (and has a license fee for anyone else).
• PyQt6 can be themed with custom color schemes and widget styles. There's lots of available themes. By default, it uses platform-appropriate themes which look completely native on Mac, Windows and Linux. But "app identity" themes can also be created.
• PyQt6 includes tons of GUI widgets for everything we could want. Many professional applications are written in it.
• It is well documented. There's a great, free e-book / documentation site at https://www.pythonguis.com/, and there's also tons of videos.
• There's also something called PySide, which is another Qt library binding for Python, but available under LGPL license instead. People say it's almost a drop-in replacement, but that it has worse docs. It seems pointless to go for it since OneTrainer is a GPL project and can use the fully mature PyQt instead.

[FurkanGozukara]

I would love GUI to move away from CustomTkInter . It is just this problematic on cloud ubuntu : https://youtu.be/hmu-7ycJ_E4

[ppbrown]

Im not sure the problems are really the gui toolkit.
I use it remotely via VNC, and it is mostly fine.
I think its more that parts of the code are not written with an interactive GUI in mind.

For example, when doing prolonged caching, the GUI updates maybe once every 5 seconds.
I'm guessing thats more to do with the backend being single threaded rather than the GUI toolkit.

That being said.. migrating toolkit is typically a huge undertaking.
I'm not a OneTrainer dev, but I woudl guess only way that will happen is if someone stepped and submitted a huge PR to implement it.

[O-J1]

Its a mixture of both, Furkan in my opinion somewhat abuses the host machine by running multiple UI instances of OT at once and since customtkinter is single threaded, 8 UIs brings it to a grinding halt.

Feats can be aspirational