Add recursive folder previews

[Mikescops]

I'm not super fan of recursivity because it can lead to lot of computation, I suggest to lock it to one sub level only. I guess this would fit most of the people configuration, what do you think @skjnldsv ?

Closes #489

[jancborchardt]

One level seems good. Does that then also mean if you go down one level, the previews of the next sublevel will automatically start being generated already? (Which would be nice, as anticipation.)

[Mikescops]

I will discuss with @skjnldsv about it.
But we don't start pre-generating inner folders

[Mikescops]

@skjnldsv let me know if you have a better idea on how to do it

[skjnldsv]

Do as we do for folder? We fetch the first 4 images but one level deeper?

[Mikescops]

I think it's good

[skjnldsv]

[skjnldsv]

Please rebase and compile :)

[BloodyIron]

I would prefer that recursivity matched the same behaviour that folder previews did before the major gallery overhaul several major versions ago. I actually used that preview recursivity to help me rapidly traverse many complex folder structures. So if this matches that previous style, I'm game for it. If not, I'd like to see that reivisited/implemented please.

[Mikescops]

Not sure what you are talking about. Please provide examples, and I recommend trying current version before.

[BloodyIron]

I've tried the current and previous versions, I've reported this in another issue# and was directed here. I don't know how my explanation isn't clear...