Unify forms, add to/remove from playlist on all platforms, UI improvements

Model/NavigationModel.swift

@@ -13,6 +13,9 @@ final class NavigationModel: ObservableObject {
 
     @Published var returnToDetails = false
 
+    @Published var presentingAddToPlaylist = false
+    @Published var videoToAddToPlaylist: Video!
+
     @Published var presentingPlaylistForm = false
     @Published var editedPlaylist: Playlist!
 
@@ -42,6 +45,11 @@ final class NavigationModel: ObservableObject {
         )
     }
 
+    func presentAddToPlaylist(_ video: Video) {
+        videoToAddToPlaylist = video
+        presentingAddToPlaylist = true
+    }
+
     func presentEditPlaylistForm(_ playlist: Playlist?) {
         editedPlaylist = playlist
         presentingPlaylistForm = editedPlaylist != nil