epi::WebInfo.web_location_hash is never updated

[awaken1ng]

Describe the bug
Location hash initialized once in egui_web::backend::AppRunner::new, and never updated afterwards.

This causes the demo not being able to switch between tabs if you open it with a link that contains fragment identifier, e.g. https://emilk.github.io/egui/index.html#demo, after clicking on one of the tabs and updating the current selection, it's then overwritten on next frame.

To Reproduce
Steps to reproduce the behavior:

1. Open https://emilk.github.io/egui/index.html#demo, #demo part is important
2. Try opening any other tab, e.g. EasyMark editor

Observed behavior
EasyMark editor tab shows up for one frame, and then reverts back to Demos tab

Expected behavior
EasyMark editor tab to open and stay

Desktop:

• OS: Linux
• Browser: Firefox 96, Chromium 97