Index file for thumbnails (do not merge yet)

[zoltanvb]

Do not merge yet - 2 tasks need to be done and at least a few index files should be present for this to make sense.

Description

Add the function to retrieve an .index-extended file for thumbnail repositories, similar to core updater. Use the list to determine if the download should be attempted or not. For now, fall back to previous method if index can not be retrieved.

Currently, this PR would bring no immediate benefits, as there are no .index-extended files in thumbnail repos. I have opened a PR to get one: libretro-thumbnails/Sinclair_-_ZX_Spectrum#14 . Either that workflow needs to be added to all thumbnail repos (copy-paste, just a lot of it), or index files can be generated centrally, but that may take a toll on the server if it is not done smartly (i.e. changed files only). Generating a new index file for the ZX Spectrum repo takes ~40 min.

Benefits if index files are populated:

• ondemand thumbnail downloads will typically result in one download per system (the index), instead of several failed ones. As thumbnails do not change very frequently, this may bring a benefit, depending on the current ratio of 404 requests (which I do not know, but would guess it is quite high).

There may be also drawbacks:

• increased memory footprint
• index file may be of significant size (2 MB for the possibly largest one)

This can also pave the way for a few improvements (not included to current code):

• if there is no suitable thumbnail, try to apply flexible naming in a way to retrieve files from the server, that have partial match, and save them with short name, so they will get shown
• if someone feels the power to do so, even implement "select thumbnail" menu
• check CRCs to determine if update of existing thumbnails is needed
• support of non-PNG thumbnails also in central repositories
• if index files are always available, thumbnail download code can be simplified by a significant amount

TODOs in any case:

• doublecheck working for Favorites/History
• remove excessive logging