Fix the label string at the detail panel.
The history of the detail panel of tasks.

This change provide a history of the detail panel, so the user could
revisit most recent 16 tasks in the reversed order of visiting.
Merge pull request #30 from ThinkerYzu/fix-parent-task-id-string

Fix the label string at the detail panel.
Merge pull request #32 from ThinkerYzu/task-visiting-history

The history of the detail panel of tasks.