feat: create dynamic asset loading

[Krzyhau]

Implement advanced asset manager which will properly cache assets and convert/load them only when necessary.

[Krzyhau]

I've tried to do this in this branch but failed due to how bad it was in terms of performance. Maybe I'm doing something stupid. Maybe checking for up-to-date versions of the file isn't really necessary. Idk.

[Krzyhau]

Mostly implemented by 5d4e188. Unsure how robust of a solution it is, would need some testing. If it's good enough, issue could be closed.