Test XMLDatabase with HashSet
New tuple classes and save method in Manager.cs
Introduce Identity.GetObjectById
Add Name property to TimeSlot and make it an Identity class
GetObjectById's id parameter should be a generic object
Manual deserialization for class-student and class-room-slot

Tuple, Tuple, Tuple...
Custom serialization for class-student and class-room-slot
Adhoc tests in Main
Fix the custom test in Main
Actually save some file in Manager.Save()
New Main
Fix a key error in Manager
Implement class screen and student screen
Add some initial code to EachStudentScreen
Introduce Confirm method in ChoiceScreen
Code formatting
Fix a test case (API mismatch)
More interface page implementation
Remove code to manage rooms and timeslots

These functionality doesn't work well with the current textbased
interface.
Use Identity.GetObjectById in TextInterface