Improve Left Main Panel Performance

[corekid96]

Hello there!
First off, thanks for making this amazing Music Player! I've been using it for years now.  

Feature Request:
I'm pretty sure this isn't a bug so I decided to create a feature request instead. When using the Left Main Panel to browse through your artists with album thumbnails enabled. The performance is very poor, even on a beefy rig like mine. I assume this is either because the album thumbnails aren't cached properly or the current method to cache is optimized for rigs that don't have much memory (e.g. unload off-screen thumbnails). Performance is perfectly fine without thumbnails enabled and I'd like this to also be the case when using thumbnails.

Perhaps an option to always keep album thumbnails loaded into memory would make scrolling buttery smooth.
(This could be disabled by default for users who care about the memory footprint of the app.)

Steps To Reproduce:
1. Enable left main panel
2. Enable thumbnails in thumbnail browser panel settings
3. Click and hold scrollbar on main panel and scroll list (requires list with many artists and available album covers)
4. Notice poor performance as app is struggling to load/unload thumbnails

Feel free to ask further questions if required. I could provide videos to showcase it better.
Cheers!