Version 0.0.2 Update Example App with a shortest Path finder

[Tolunay Ata]

The example app is lacking examples.

The task would be to put in a button with the Label Find shortest Path in main.dart.
On press a new view would be opened with three preset graphs that can be chosen from a DropdownButton. A start node and a end node should be selectable from the graph. When confirmed the shortest path should be rendered on the screen below.

This issue is part of #4