topcat: refactor progress bar updating

Use new class Progresser, which handles out-of-EDT updating of progress bars.
topcat: log progress for more plot operations

Now identifying a (clicked-on) point and turning blob-selected points into
a subset, which can both be time-consuming, both have their progress
logged with the plot progress bar.

Did a bit of other other refactoring and tidying up at the same time.
topcat: put sketch toggle action in toolbar
topcat: click-to-center now logs progress