Support live reloading browser DOM UI modifications

[RobertCraigie]

It's surprising that a snippet like this

glide.autocmds.create("ConfigLoaded", () => {
  document.head!.appendChild(DOM.create_element("style", {
    textContent: css`
      #TabsToolbar {
        visibility: collapse !important;
      }
    `,
  }));
});

will result in the <style> element being created every time :config_reload is ran.

we should keep track of any modifications to the browser Document from the config and try to revert them when the config is reloaded.

however this will be quite tricky to get fully correct, and I'm not quite sure what to about potential memory usage increases, as if you frequently modify the UI, then we'd keep collecting mutations to reverse and build up a large array...

there's maybe a solution here where we can slice out (contiguous only?) redundant modifications, e.g. [add <a>, update <a>, delete <a>, add <span>] could be transformed to just [add <span>], as the first three entries are redundant.