Revert "Merge pull request #362 from mtwebster/thumbnail-try-again"

This reverts commit eb2279c55f1ab4e5827c970e35e159635d7ee4a1, reversing
changes made to 4df4e1ee42d77618b462e5b1afb821e3ed287ce4.
Revert "Merge pull request #350 from mtwebster/thumbnail-fix"

This reverts commit 9ca75e3cc172c124f29d525d4ed4a7b914d70cfa, reversing
changes made to 8b01eb5ab0db88a6f516ba6c22e393c2f62f884f.
Add option to start in dual-pane mode.