Browser UI features

These are features that could be implemented in the Blitz browser UI (`/apps/browser`).

### Browser UI Backlog

- [ ] Menu items:
- [x] Toggle incremental layout
- [x] Toggle element inspector
- [ ] Toggle element outlines
- [x] Capture AnyRender scene
- [x] Capture screenshot
- [x] View source
- [x] Clear HTTP cache
- [x] Built-in pages
- [x] Custom start page
- [x] Generic error page?
- [x] 404 page
- [x] FPS counter
- [x] Status bar
- [ ] File downloads (`Content-Disposition: attachment`)
- [ ] Context menu (right-click menu)
- [ ] "Image document" support (if an image is navigated to directly as the top-level URL then it should be rendered as an image rather than being parsed as a document!)
- [ ] Styling
- [ ] Dark mode
- [ ] Keyboard shortcuts (Ctrl on Windows/Linux, Cmd on macOS)
- [ ] T for new tab. W for close tab.
- [ ] + / - / 0 for zoom (should remove from `blitz-shell`)
- [x] Tabs (multiple embedded documents)
- [ ] https://github.com/adamschwartz/chrome-tabs
- [ ] Focus URL bar when creating a new tab
(may need support for dispatching Dioxus events in response to programmatic focus)
- [ ] Animated tab creation
- [ ] Draggable: https://gist.github.com/alexreardon/9ef479804a7519f713fe2274e076f1f3
- [x] Favicon support
- [x] History + URL suggest (nucleo crate)
- [ ] Find in page
- [ ] Ad blocking using https://github.com/brave/adblock-rust
- [ ] Reader mode