While the methods to perform an operation with busy indication are
already a great help to write responsive UI there is one case missing
where a (Completable)Future is already present, this can for example be
used to cache some heavy work.

This now adds a new BusyIndicator.showWhile(Future) method that covers
the use case of an externally provided future computation so the caller
can be sure that while it takes place the UI shows an indicator.

Beside that the Snippet30 was enhanced to demonstrate the new function.
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Co-authored-by: Ed Merks <Ed.Merks@gmail.com>
Co-authored-by: Sebastian Zarnekow <Sebastian.Zarnekow@gmail.com>