Respect version skipping in win_sparkle_check_update_without_ui

Use UpdateChecker class in win_sparkle_check_update_without_ui() in the
same way that the initial update does, so that silent check respects the
"skip this version" state.

Rationale: manual check should report the latest version even if
previously ignored, but win_sparkle_check_update_without_ui() is more
likely than not used as a replacement for the automatic checks and so
shouldn't annoy people.

See #32.