Added very basic checklist functionality

This is absolutely subject to change. This is just a trial run to get advice on the checklist implementation so far.

src/checklist_view.py

@@ -0,0 +1,94 @@
+"""
+考えろ、バカ：
+    - Checklists will have a tree structure.
+        - The parent nodes are checklists.
+        - Leaf nodes are checklist items.
+        - If a parent task's status is changed to completed, all of its children will be changed as well.
+        - If all of a parent task's children tasks are completed, it will also be automatically completed.
+        - If one of a completed parent task's child tasks is un-completed, the parent task will be as well.
+
+    - Until it's discussed more, I'll treat the schedule variable as the number of days until which you want the list
+      to reset.
+
+    CURRENT SETBACKS:
+        - There might be unfortunate times whenever you accidentally complete a parent task, subsequently completing
+          all of the subtasks. In which case, it might be impossible to auto 'un-complete' the subtasks.
+"""
+
+
+class ChecklistItem:
+    def __init__(self, task=None):
+        self.task = task
+        self.parent = None
+        self.children = []
+        self.completed = False
+
+    def add_child(self, sub_task):
+        sub_task.set_parent(self)
+        self.children.append(sub_task)
+
+    def mark_task(self):
+        self.completed = True
+
+    def unmark_task(self):
+        self.completed = False
+
+    def set_parent(self, parent):
+        self.parent = parent
+
+    def get_task(self):
+        return self.task
+
+    def get_completion(self):
+        return self.completed
+
+    def get_parent(self):
+        return self.parent
+
+
+class Checklist:
+    def __init__(self, schedule=0):
+        self.root = ChecklistItem()
+        self.schedule = schedule
+
+
+def print_checklist(root):
+    for item in root.children:
+
+        temp = root
+        while temp.get_parent() is not None:
+            temp = temp.get_parent()
+            print("    ", end="")
+
+        if item.get_completion():
+            print("x " + item.get_task())
+        else:
+            print("o " + item.get_task())
+
+        print_checklist(item)
+
+
+if __name__ == "__main__":
+    test_checklist = Checklist()
+
+    """
+    As a proof of concept, I'll make a checklist similar to the following, x is complete, o is incomplete:
+    Backpack Checklist
+    o back pocket
+        x laptop
+        o げんき third edition Japanese textbook and workbook with bonus learning exercises
+    o front pocket
+        o pens
+        o headphones
+    """
+
+    test_checklist.root.add_child(ChecklistItem("back pocket"))
+    test_checklist.root.children[0].add_child(ChecklistItem("laptop"))
+    test_checklist.root.children[0].children[0].mark_task()
+    test_checklist.root.children[0].add_child(ChecklistItem("げんき third edition Japanese textbook and workbook with "
+                                                            "bonus learning exercises"))
+    test_checklist.root.add_child(ChecklistItem("front pocket"))
+    test_checklist.root.children[1].add_child(ChecklistItem("pens"))
+    test_checklist.root.children[1].add_child(ChecklistItem("headphones"))
+
+    print_checklist(test_checklist.root)