[leveller] Leveller 2.6 alpha build 2538 available

Changes are:

- Mem block free bug after copying scene to clipboard; fixed.
- Errors on unloadable plug-ins explained better, will catch
  situations, e.g., where a release build plug-in is accidentally
  linked against debug runtimes such as mfc42d.dll and shipped
  to a release-only environment.
- Startup logfile formatted nicer, events ordered better, and
  elapsed time included in report.


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