Compare heap dump selector does not see already opened heap dumps

[thurka]

Describe the bug Compare heap dump selector does not see already opened heap dumps.

To Reproduce Steps to reproduce the behavior:

1. Prepare two heap dumps and put them to different directories.
2. Open both heap dumps in VisualVM via File->Load...
3. Invoke 'Select heap dump to compare' dialog.
4. Already opened heap dump is not available in dialog, you have to use 'From file'.

Expected behavior 'Select heap dump to compare' dialog should offer already opened heap dumps.