Further improve asset library install experience

[TheDuckCow]

Right the asset library install still has a couple problems:

• Add .gut_editor_config.json to the gitattributes file with the flag of export-ignore so it's not included in the Asset Lib default install list.
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• If the user has installed the plugin for the first time, custom icons appear in Add Node menu, but they won't appear as the icon right when they've been added into a scene.
  • If the user reloads their project closes and reopens the scene though, it starts working. Seems to be a godot issue, as even pre-running or copying over .import settings doesn't seem to assist.
• Remove the broken gif (that can be edited without a further release). It's a bit confusing when editing the asset, with the different tick box options included, but hopefully next time it'll work better. The gif works on the asset page, but appears broken inside Godot itself.
• Per below, there's also a layout problem in the asset library page inside Godot itself. Not sure if we can do anything about this, this happens to other assets in the lib too. Could be that playing with different aspect ratios could help (or perhaps having a consistent aspect ratio / image size across the demo images).
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[TheDuckCow]

Trying to mitigate custom icons not appearing until restart: I tried a workflow where I copied over the .import folder with the plugin on download, but that still had the same issue. In fact, this is a known godot issue, where you just need to close and re-open the scene: godotengine/godot#35896