ev-window: Don't attempt to load non-seekable files directly

In a regression from commit 6cb4a630 ("shell: Don't copy whole files from remote shares"), on some backends (including ftp and archive), which don't allow for seeking, evince unsuccessfully attempts to open documents directly, instead of downloading first. Keep the existing logic to presumably still optimize cases where seeking is allowed, but make sure to first download files where seeking is not possible.

Fixes: #1971