No item selection and proper keyboard navigation in the web demo

[vi]

Describe the bug

List items should typically be selectable and keyboard actions should affect that selection.

To Reproduce
Steps to reproduce the behavior:

1. Open the web demo: https://emilk.github.io/egui/#demo
2. Activate "Drag and drop" and "Scrolling" windows
3. Click onto some list item in one of the visible lists
4. Press "down" keyboard button

Expected behavior

Selected item is highlighted somehow.
Pressing down key moves makes lower item highlighted instead.

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It is in general tricky to access all features of the web demo using keyboard only. A lot of Tab / Shift+Tab and other bugs (i.e. uncloseable combobox) gets encountered this way.

[emilk]

What is needed in order to accomplish keyboard navigation has been discussed in more detail in #1250, so I'll close that in favor of #1250