BlockFile.cpp does not depend on MissingAliasFileDialog.cpp ...

... instead use a hook function to call back when it is discovered that a
block file is missing.

This frees three files from dependency cycles

src/MissingAliasFileDialog.cpp

@@ -131,3 +131,12 @@ namespace MissingAliasFilesDialog {
    }
    
 }
+
+// Arrange callback from low levels of block file I/O to detect missing files
+static struct InstallHook{ InstallHook() {
+   auto hook = [](const AliasBlockFile *pAliasFile){
+      if (!MissingAliasFilesDialog::ShouldShow())
+         MissingAliasFilesDialog::Mark(pAliasFile);
+   };
+   BlockFile::SetMissingAliasFileFound( hook );
+} } installHook;