Add thumbnail support for Nextcloud/OwnCloud

[skjiisa]

Currently the only WebDAV server that this library supports fetching thumbnails from is yandex, but Nextcloud and OwnCloud support thumbnails too, using a different method.

To fetch a thumbnail from Nextcloud, download the file index.php/core/preview.png with the file parameter set to the file path, the x and y parameters set for dimensions, and optionally a=1 and mode=cover. a=1 will crop the image to fill the dimensions specified whereas omitting it will keep the aspect ratio, making the result equal-to or smaller-than the specified dimensions. I haven't been able to figure out what mode=cover does, but that's what the official Nextcloud app uses.

Example: https://nc.example.com/index.php/core/preview.png?file=Media/image.png&x=128&y=128&a=1&mode=cover

Currently thumbnailOfFile and thumbnailOfFileSupported check if the WebDAV host contains dav.yandex., though Nextcloud and OwnCloud servers can have any domain name, so I'm not sure how it could be checked if the server is Nextcloud or OwnCloud in order to use this thumbnail-fetching method, or to check if thumbnails are supported.