Add "Stop" Toolbar button which aborts e.g. filtering

[jens-maus]

Originally on 2010-03-06 00:49:47 +0100

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As soon as we have implemented the user-definable toolbars we ought to add a "Stop" toolbar button in case there is some action in progress which might be able to be aborted (e.g. filtering, index scanning etc.)

[jens-maus]

Originally on 2014-05-12 10:10:31 +0200

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Now that we have proper multithreading for a while now, I feel we urgently need such a Stop toolbar button which automatically appears as soon as there is something to be aborted (filtering, index scanning, etc.) and will abort everything as a last resort.