Improve demo

Select manifest URL on focus for improved usability.

index.html

@@ -105,6 +105,7 @@
 				id="manifestUrl"
 				placeholder="Enter a IIIF manifest URL"
 				aria-label="Manifest URL"
+				onfocus="event.target.select()"
 			/>
 			<button type="submit" class="button">Add</button>
 			<button type="reset" class="button" onclick="removeAllInstances()">Remove all</button>