Include library of standard icons

### What is the problem or limitation you are having?

Libraries like Material Design include several platform specific icons. Things like un/filled stars for ratings. Toga only provides the default icon (which is adorable, btw)

### Describe the solution you'd like

The ability to do something like `icon=toga.Icon.star-filled`

### Describe alternatives you've considered

downloading entire libraries to my project folder

### Additional context

briefly discussed on Discord

Start by examining the existing toga.Icon API and how platform-specific icons are represented. Define the scope of the standard icon library, including names such as star-filled, and verify that the requested icons are available consistently across supported platforms.

Written by the indexing model from the issue text.