Show xapian update progress when it changes

Summary:
Sometimes update-axi is slow to start, and in the time between
`xapianUpdateStarted` is emitted and the first `xapianUpdateProgress`
the timer expires and the progress bar is hidden. Now we show it every
time we get a progress update.
Also, increase the timeout to 30s before hiding.

Reviewers: sitter

Reviewed By: sitter

Differential Revision: https://phabricator.kde.org/D10717