Added: Support for Enhanced Tree Node Icons

[Sewer56]

Review after:

• Changed: Don't use custom join in GamePathNode & Update Paths Library #1067
• This PR was rebased on that PR, as it depends on it.

This PR adds support for displaying file icons in View Mod Files and Advanced Installer.
I.e. Start of '2nd phase' of Viewing/Diffing files.

Nothing more, nothing less.
As demo'd at the end of last week, but with AdvancedInstaller, too!

Checks off another box in:

• Mod Contents file tree view Refinements #1048

I also made FileTreeNodeDesignViewModel inherit from FileTreeNodeViewModel, saving a few lines of code. Originally this was a piece of feedback on the original View Mod Files PR. At the time it wasn't as easily possible, as we stored data a bit differently. But now, there is no problem.

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Note: This is marked as a draft because it's blocked as the PR on NexusMods.Paths is still open, and we don't have a new package.

[github-actions]

This PR conflicts with main. You need to rebase the PR before it can be merged.

[Al12rs]

@Sewer56 rebase and mark as ready for review and we can get this merged as well

[github-actions]

This PR doesn't conflict with main anymore. It can be merged after all status checks have passed and it has been reviewed.