New variant for "show thumbnails": "remote if provided"?

Now that sh_thumbnails are supported, the preferences setting of "show thumbnails" (currently: "never", "local files only", "always") could gain a new option: "remote if provided" ("when effortless"? We'll need good wording too).

In that setting, thumbnails will be show for remote files only if the remote provides them in form of sh_thumbnails already. This should also make a good candidate for a new default (currently: "local files only"), as it follows its intention: Do what can be done without creating extraneous amounts of network traffic. (The old "local only" option still makes sense, for if you're on metered bandwidth or a slow link, even transferring the thumbnails can be too much to. Personally, I'd be just as happy with Thunar using sh_thumbnails whenever available, but I imagine there are cases when that's not desirable).

Alternatively, it may make sense to split up the "Only show thumbnails for files smaller than" to local and remote files, and merely consider any existing .sh_thumbnail's size when one is available, such that a setting of "if smaller than 512KiB" would practically always catch on when shared thumbnails are provided, and not otherwise.

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As for the current state, I can't test that properly: the xfce-test environment I've tested general sh_thumbnails with doesn't ship gvfs, and when I install gvfs and gvfs-backends to get to my davs:// test server, I don't get any thumbnails at all (whereas the current Debian sid version happily produces thumbnails). Anyway, as this'd need additional user choices, it can probably be discussed independent of currently observed behavior.