Gather balance info hashes into one struct guarded by an accessor...

Doing BalanceInfoDel in this delayed fashion will be needed when the other
call to it is eliminated, as shared_ptr simplifies the management of BlockFile
and makes the many calls to Ref and Deref disappear.

src/BlockFile.h

@@ -64,6 +64,8 @@ class PROFILE_DLL_API BlockFile /* not final, abstract */ {
    BlockFile(wxFileNameWrapper &&fileName, sampleCount samples);
    virtual ~BlockFile();
 
+   static unsigned long gBlockFileDestructionCount;
+
    // Reading
 
    /// Retrieves audio data from this BlockFile