Add an "about" page for triangulate-rs.

[ShampooDeng]

The "about" page should be a pop out window running aside from the main window. And "about" page should contain the author information, project repository url, and some other links related to the app.

After a quick search:

• Q: Multiple top-level windows? emilk/egui#552
• Multiple native windows emilk/egui#1044

egui seems able to create a multiple window app. Examples may be found in:

• egui example
• https://github.com/vivlim/egui-glow-multiwin