Fixed a self-update bug that could clear your temp folder

If you updated WhatCable from inside the app, its cleanup step could delete
the contents of your Mac's shared temporary folder, not just WhatCable's own
leftovers. That meant temporary files belonging to other open apps could be
removed too. This release fixes it: the updater now only ever deletes its own
download folder.

Fixed

• In-app update no longer touches the shared temp folder. The updater's
  cleanup was pointed one level too high and removed the whole temporary
  directory after a successful update. It now removes only the per-update
  download folder it created. This also tidies up a folder that used to be
  left behind after a successful update. Thanks to @k100shn for spotting it
  and writing up exactly where it went wrong.

Only the in-app Update button was affected. Updating with Homebrew or by
downloading from whatcable.uk was never at risk.