Enable focus for tabs and display

You can now switch tabs from the keyboard using the usual radio button
interaction.

app-proto/inversive-display/src/main.rs

@@ -482,7 +482,7 @@ fn main() {
                     }
                 }
                 div { "Mean frame interval: " (mean_frame_interval.get()) " ms" }
-                canvas(ref=display, width="600", height="600")
+                canvas(ref=display, width=600, height=600, tabindex=0)
                 div(ref=gen_controls) {
                     label(class="control") {
                         span { "Sphere 0 depth" }