Put navigation icons in the centre of the viewbox (#295)

* Redraw performance icon
  * It looks more like a lightning bolt now.
* Redraw changelog icon
  * It looks like a plus-minus sign now.
  * This better represents changes than the previous one (a bulleted list).

Author: tfpf

docs/assets.zip

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:0ec8666e49fd0ac7e7801f133a45a2868cb169832a223afccff4b6e22308a672
-size 689177
+oid sha256:8a4dedef9e27c97e2462b92434d69ae999278deb9c2a01f501e26a85ad6d2026
+size 689183