Add folder display capability

[thegail]

Pretty much what it sounds like. I matched the Discord UI very closely. Opportunities for future improvement/other notes:

• Folders with only one server are displayed as single servers...the API is not very well structured in this area and I didn't find a way to differentiate between folders containing only one server and a server which isn't in a folder
• Again, the API in this area is very unclear, so I'm not exactly whether ungrouped folders, which were never added to a group, but had their order rearranged, will display in the correct order. If they don't, they should just display above all of the folders. This is something that may be desirable to test later.
• I created two new data structures, ContentView.ServerListItem and ServerFolder.GuildFolder. I didn't really know where to put these, since the project's current data model was unclear to me. This is something that could be rather painlessly fixed before merging.
• I added popovers to display the names of folders. This causes serious choppiness while scrolling through large numbers of folders (blocking the animation thread?) but since that scenario is relatively uncommon, and there is no other way to view the name of the folder, I found the tradeoff acceptable. This could also be changed before merging.
• I had to make some rather odd changes to get this project to build on my system (including commenting out UserSettingsView.swift:35 and LoadingView.swift:37). I did my best to make sure none of these changes got committed (including changes to Swiftcord.pbxproj). Because of this, I also don't know for sure whether my changes will build properly on a CI server or anyone else's machine, though I suspect they should.
• I wasn't able to get a good folder "drawer" opening/closing animation. The server icon previews and folder icon slides up and down as in vanilla Discord, but I wasn't able to get other servers to "move out of the way" smoothly, and they just ended up jumping to the correct position. This resulted in the animation looking half-baked, so I decided to remove it. Maybe someone could work some SwiftUI magic to get this fixed.
• Animated mini-icons inside the folder preview always animate, not just on hover
• The mini-icons inside the folder preview look awful on a low res screen, at least compared to vanilla Discord. I tried using SwiftUI's antialiasing (which is still explicitly enabled on those mini-icons), but it didn't really help
• In order to do this, I made some modifications to BetterImageView, allowing a consumer to pass an (Image) -> Image closure to make modifications to the original image. This is backwards compatible (at least for all the existing usages in the codebase) and an additive change.

I'm happy to DM screen recordings of my UI, in case you don't want to build my changes. I just don't really want the names of all the servers I'm in out in the wild.

[cryptoAlgorithm]

Nice PR! I have some suggestions though:

Animated mini-icons inside the folder preview always animate, not just on hover

Could this be made such that the icons only animate when the folder is hovered? That would improve performance I suppose.

Folders with only one server are displayed as single servers...the API is not very well structured in this area and I didn't find a way to differentiate between folders containing only one server and a server which isn't in a folder

Folders can actually have one server. It would be great if this case could be handled. However...

user_settings is a depreciated key and actually is already removed in the latest client versions. Swiftcord will also no longer receive this key in the future, so it would be prudent to move to the user_settings_proto (which I can help you adopt once this is merged)

[cryptoAlgorithm]

That commit contains major clean-ups and animates server icons on hover only