A-FullCommitMessage: Implement a better task GUI

Implement a task calendar and creation workflow
Add a graphical task-management workflow that allows users to:
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Create todo, deadline, and event tasks through a dialog
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View scheduled tasks within an inclusive date range
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Mark and unmark tasks directly from the calendar view
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Return to the date-range selection screen
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Display validation and parser errors in the interface
Expose task completion and scheduling data through accessors, and provide a read-only task list view for the GUI. Preserve task persistence through the existing storage and parser components.