Supports app versions with explicit build versions

AltStore will now consider an update available if either:

* The source’s marketing version doesn’t match installed app’s version
* The source declares a build version AND it doesn’t match the install app’s build version

The installed app matches an app version if both maketing versions match, and the build versions match (if provided by the source).

AltStoreCore/Model/AppVersion.swift

@@ -13,9 +13,17 @@ public class AppVersion: NSManagedObject, Decodable, Fetchable
 {
     /* Properties */
     @NSManaged public var version: String
+    
+    // NULL does not work as expected with SQL Unique Constraints (because NULL != NULL),
+    // so we store non-optional value and provide public accessor with optional return type.
+    @nonobjc public var buildVersion: String? {
+        get { _buildVersion.isEmpty ? nil : _buildVersion }
+        set { _buildVersion = newValue ?? "" }
+    }
+    @NSManaged @objc(buildVersion) public private(set) var _buildVersion: String
+    
     @NSManaged public var date: Date
     @NSManaged public var localizedDescription: String?
-    
     @NSManaged public var downloadURL: URL
     @NSManaged public var size: Int64
     @NSManaged public var sha256: String?
@@ -51,6 +59,7 @@ public class AppVersion: NSManagedObject, Decodable, Fetchable
     private enum CodingKeys: String, CodingKey
     {
         case version
+        case buildVersion
         case date
         case localizedDescription
         case downloadURL
@@ -71,6 +80,8 @@ public class AppVersion: NSManagedObject, Decodable, Fetchable
             let container = try decoder.container(keyedBy: CodingKeys.self)
             
             self.version = try container.decode(String.self, forKey: .version)
+            self.buildVersion = try container.decodeIfPresent(String.self, forKey: .buildVersion)
+            
             self.date = try container.decode(Date.self, forKey: .date)
             self.localizedDescription = try container.decodeIfPresent(String.self, forKey: .localizedDescription)
             
@@ -94,6 +105,26 @@ public class AppVersion: NSManagedObject, Decodable, Fetchable
     }
 }
 
+public extension AppVersion
+{
+    var localizedVersion: String {
+        guard let buildVersion else { return self.version }
+        
+        let localizedVersion = "\(self.version) (\(buildVersion))"
+        return localizedVersion
+    }
+    
+    var versionID: String {
+        // Use `nil` as fallback to prevent collisions between versions with builds and versions without.
+        // 1.5 (4) -> "1.5|4"
+        // 1.5.4 (no build) -> "1.5.4|nil"
+        let buildVersion = self.buildVersion ?? "nil"
+        
+        let versionID = "\(self.version)|\(buildVersion)"
+        return versionID
+    }
+}
+
 public extension AppVersion
 {
     @nonobjc class func fetchRequest() -> NSFetchRequest<AppVersion>
@@ -103,6 +134,7 @@ public extension AppVersion
     
     class func makeAppVersion(
         version: String,
+        buildVersion: String?,
         date: Date,
         localizedDescription: String? = nil,
         downloadURL: URL,
@@ -113,6 +145,7 @@ public extension AppVersion
     {
         let appVersion = AppVersion(context: context)
         appVersion.version = version
+        appVersion.buildVersion = buildVersion
         appVersion.date = date
         appVersion.localizedDescription = localizedDescription
         appVersion.downloadURL = downloadURL