plugin XEP-0264: replace `SIZE_FULL_SCREEN` by `SIZE_BIG`: `SIZE_FULL_SCREEN` may result in large files, and BoB is currently used to transfer them, resulting in potentially large stanza. As a result, Libervia profile can be disconnected from server if stanza max size is reached. As a temporary workaround, `SIZE_BIG` is now the maximum size used. This should be fixed in 0.9 with a proper way to transfer large thumbnails (i.e. not BoB).