Represent a completed Task with a tick icon

* Generate a tick icon via Line canvas instruction

* Store the line's points in the TickIcon to make use of the observer
* pattern and keep the icon's form relative to the checkbox

* Show and hide the icon by adding and removing it's widget from the
* Checkbox widget respectively
Remove Menu, Open the app with the main TasksScreen
Store tasks in tasks.txt file

Add a function to load from an open file. The function will be recursively
loading the task tree in the future.

Tick Icon displays increctly on load. It will be replaced with an image in the
future.