Bug 330 (P2) - Fix an issue where multiple projects would cause the warning dialogs to pop up above the wrong project.

I refactored the code into AudacityApp with a new timer.  This is provisional pending discussion - if it is decided that it should go somewhere else I will move it.

src/blockfile/PCMAliasBlockFile.cpp

@@ -109,8 +109,8 @@ int PCMAliasBlockFile::ReadData(samplePtr data, sampleFormat format,
       mSilentAliasLog=TRUE;
       
       // Set a marker to display an error message for the silence
-      if (gAudioIO)
-         gAudioIO->MarkAliasedFilesMissingWarning();
+      if (!wxGetApp().ShouldShowMissingAliasedFileWarning())
+         wxGetApp().MarkAliasedFilesMissingWarning(this);
       return len;
    }