feat: apply loading message delay for directories

[joelim-work]

• Fixes Add delay before showing directory loading message #2407

This PR changes the way delays are handled when displaying the loading... message.

dir and reg objects contain a loadTime variable, which conceptually represents the time the object was last created or updated. For objects that represent real directories/previews, this is used to check for modifications by comparing against mtime. For placeholder objects representing directories/previews that are in the process of being loaded, this can be used to measure how long it has been loading, and whether or not enough time has passed to display loading... without causing flickering.

[CatsDeservePets]

Scrolling through directories feels much smoother now.

[CatsDeservePets]

Interesting. I never noticed before, but volatile previews only ever displayed loading... once, even though they have to load every time.

This is fixed now.

[joelim-work]

It is because of this part of the code, which marks the preview as reloading entirely from scratch if it is volatile:

if volatile && r.volatile {
	if !gOpts.preload {
		r.loadTime = time.Now()
		r.loading = true
	}
	nav.previewChan <- path
}

One thing I dislike about the initial implementation of previews is that it conflates 'loading' with 'displaying'. The preload option separates these two steps:

• Loading is performing any prerequisite work (e.g. generating thumbnails from a video), and then loading the preview into a reg object.
• Displaying is taking that reg object and physically displaying it in the UI. For text and sixel previews, the preview data is stored directly in the reg object, but volatile previews (e.g. Ueberzug images) have a flag set which then delegates the actual work to the previewer script.

This does mean now that the loading... message now only applies for the loading step, but in theory the displaying step should be fast and not require such a message.