Bug2531: need progress indication when discarding undo history...

... You may see this also when abandoning lots of redo history, and not only
when doing compaction.

If in compaction you discard much undo and also much redo, you may see two
progresses.

It's debatable whether this might have been better implemented by reuse of
ProjectFileIO::DeleteBlocks instead, putting callbacks to a progress indicator
in the function InSet().  But I wanted to avoid more dependency onto
ProjectFileIO.

Doing real work in DeleteBlocks() is supposed to happen only if there is a bug
elsewhere that allowed orphans.  So, still no progress indicator there.

src/SampleBlock.h

@@ -135,6 +135,13 @@ public:
    /*! @return ids of all sample blocks created by this factory and still extant */
    virtual SampleBlockIDs GetActiveBlockIDs() = 0;
 
+   //! Type of function that is informed when a block is about to be deleted
+   using BlockDeletionCallback = std::function< void(const SampleBlock&) >;
+
+   //! Install a callback, returning the previously installed callback
+   virtual BlockDeletionCallback SetBlockDeletionCallback(
+      BlockDeletionCallback callback ) = 0;
+
 protected:
    // The override should throw more informative exceptions on error than the
    // default InconsistencyException thrown by Create