Adds exporter for list of assets to XML

[notjosh]

Hi!

I've been using AssetStudio, and it's been great. But because assets are exported with filenames as the asset name, it was impossible to follow references via "path ID" from other assets.

So, I've added an XML exporter to the Export menu ("All", "Selected", and "Filtered"), which will dump a list that looks like...

<?xml version="1.0" encoding="utf-8"?>
<Assets filename="D:\path\to\assets.xml" createdAt="2021-03-21T18:16:49">
  <Asset>
    <Name>PermanentManager</Name>
    <Container></Container>
    <Type id="114">MonoBehaviour</Type>
    <PathID>5593</PathID>
    <Source>D:\path\to\asset\level0</Source>
    <Size>32</Size>
  </Asset>
  <Asset>
    <Name>TextTranslation</Name>
    <Container></Container>
    <Type id="114">MonoBehaviour</Type>
    <PathID>5594</PathID>
    <Source>D:\path\to\asset\level0</Source>
    <Size>536</Size>
  </Asset>
  <Asset>
    <Name>EventSystem</Name>
    <Container></Container>
    <Type id="114">MonoBehaviour</Type>
    <PathID>5595</PathID>
    <Source>D:\path\to\asset\level0</Source>
    <Size>52</Size>
  </Asset>

  <!-- ...etc -->
</Assets>

The only issue I really see right now, is that It doesn't match duplicate names (that have files like SomeName #1234.ext when exported), so if a project has a lot of duplicates it's still a little hard to follow. But, it's been helpful for me, so might be useful for others too :)

In future, I'd love to have the metadata directly in the preview(+export) and/or making the m_pathID entries clickable that jump straight to the asset.

I'd add CSV/JSON but didn't want to add dependencies, but happy to do so if that's helpful also.

Addresses #517 (somewhat).

[Perfare]

This is very good, there should be many people who need this, thank you for your help.
Regarding json, AssetStudioGUI has added the Newtonsoft.Json library, you can use it directly.