[guilib] optimized preloader, add "unloaddelayed" attribute

[axbmcuser]

Hi :-)

heads-up:
I will edit this PR later with some more info

• This PR optimizes the guilib container preloader code
  ("preloaditems" is mostly used with skin panel controls and wall views in skins)
• It also adds an fully optional (false by default) "unloaddelayed" attribute which is able to fix premature unloading of top most wall view row in Estuary and similar skin views like the walls in Confluence ZEITGEIST and other skins.
  This new attribute easily enables skin devs to fix the visual "ui glitch" on such wall views without affecting anything else

Will see if i can provide some examples of the visual glitch later.

[avdvplus]

I need more info on this one. When you have a chance please provide more background and examples of this problem and the proposed fix. Thank you.

[axbmcuser]

@avdvplus

Sure thing.
Will follow up more if needed - just a bit short on time at the moment. :-)

But here already a quick screen capture snippet:

https://upload.disroot.org/r/NXg52r6E#N2NamcZXl9mnrq6AVhauEbW/PVtGBvNOToPDKM7NZkc=
pw: debugme

Look at the first row of posters which is by design a bit out of view.
Estuary, Confluence ZEITGEIST and other skins do walls like this where the first row is mostly out of view because it's a visually pleasing concept.

On Estuary the problem does really not stick out because the animation starts quicker (other tween curve used) and a dark shadow is used at the top which kinda obfuscates the visual issue a bit.
But i've also seen the top row flickering when scrolling down in Estuary. Sadly no screen capture of that right now. :-)

Long story short:

The fully optional & disabled/false by default "unloaddelayed" attribute makes it possible for skin devs to use such common, refined and clean wall views without unloading the top row too soon - which fixes the visual issues in these situations.

Maybe important to affirm:

When the new optional attribute is not set, nothing changes.

Things stay as is for all skins who don't even know about the new attribute yet.