Basic tab strip for the minibrowser (#33100)

This implements a simple tab system for servoshell:
- The egui part uses the built-in SelectableLabels components and
  display the full tab title on hover.
- WebView structs now hold all the state for each WebView. When we
  need "global" state, we return the focused WebView state, eg.
  for the load status since it's still global in the UI.
- New keyboard shortcut: [Cmd-or-Ctrl]+[W] to close the current tab.
- New keyboard shortcut: [Cmd-or-Ctrl]+[T] to create a new tab.
- The new tab content is loaded from the 'servo:newtab' url using a
  couple of custom protocol handlers.

Signed-off-by: webbeef <me@webbeef.org>

components/shared/net/response.rs

@@ -184,6 +184,10 @@ impl Response {
         }
     }
 
+    pub fn network_internal_error<T: Into<String>>(msg: T) -> Response {
+        Self::network_error(NetworkError::Internal(msg.into()))
+    }
+
     pub fn url(&self) -> Option<&ServoUrl> {
         self.url.as_ref()
     }