Swift UI initial start

[stevenzeck]

This is an initial start of rewriting the TestApp using SwiftUI. Not much works at the moment, but I wanted to start putting things together so others can contribute.

What's been done

1. Added a tab bar with tabs for Bookshelf, Catalogs, and About. Used icons from Apple for them.
2. Content for the About screen. The layout isn't optimized precisely yet though
3. Catalogs tab has the 3 default OPDS feeds (can't delete these yet)
4. Bookshelf retrieves the list of books from the database and displays the cover image, title, and authors. Nothing else at the moment. The layout for this also isn't optimized yet. You can test this by using a branch other than swiftui and downloading a book, then switch to the swiftui branch
5. Add a new function, parseURL, to async parse OPDS feeds. The function name is the same as the current one, just async, so no breaking changes
6. Adopted the use of "ViewModel" for each screen. Whether this is a good architectural change or not remains to be seen.
7. Sheet view for adding a book from the Bookshelf tab (non-functional right now, only UI)
8. Sheet view for adding an OPDS feed from the Catalogs tab. Validates that the title and URL are there before allowing saving, then validates the URL before adding. If it fails, it shows an alert.

What you need to do

1. After you checkout this branch, rerun make dev
2. To run the TestApp with SwiftUI, comment out @UIApplicationMain in AppDelegate.swift and uncomment @main in TestApp.swift

[stevenzeck]

@mickael-menu can we keep this or refactor to however you want it? It will be used at some point

[mickael-menu]

Erm sorry, I missed your comment before merging the PR. This should be easy to reintroduce when we'll use it.

[mickael-menu]

Thank you for kickstarting the SwiftUI rewrite!