Fix hidden category/channel bug

[cyberme0w]

Hi!

I saw #368 and decided to take a bit of a closer look at it. I set up a server like this, which I think emulates the situation described in the issue close enough:

• private-category (visible to admin only)
  • role1 (visible to role1 only)
  • admin (admin only)
• public-category (visible to all)
  • role1 (visible to role1 only)
  • admin (admin only)
• empty-public-category (visible to all)
  • (no channels added here)

Previously, this resulted in the following GuildTree:

• public-category
  • role1

With this fix, the GuildTree looks like this for the role1 member:

• private-category
  • role1
• public-category
  • role1

This matches how the web client displays them, too.

The issue was that categories were not being drawn if the user did not have view channel permissions for them. However, a category can be hidden and still have user-visible channels, which should be drawn in the guild tree.

With this fix, the createChannelNodes() function now has three steps as follows:

1. Draw all top-level, non-category channels
2. Draw categories that have at least one visible channel (regardless of the categories' own visibility)
3. Draw channels inside (visible) categories if they are visible to the user

Cheers!

[epetousis]

Fix definitely works, thanks so much!