Refactor code, create abstract class ExternalUpdater

(Hopefully) this makes implementing updaters using external libraries easier on other platforms. To implement an updater on a new platform, create a new class that implements the pure virtual methods from `ExternalUpdater` and add code in the `UpdateChecker` initializer to initialize the new class.

launcher/updater/ExternalUpdater.h

@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-3.0-only
+/*
+ *  PolyMC - Minecraft Launcher
+ *  Copyright (C) 2022 Kenneth Chew <kenneth.c0@protonmail.com>
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, version 3.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef LAUNCHER_EXTERNALUPDATER_H
+#define LAUNCHER_EXTERNALUPDATER_H
+
+#include <QObject>
+
+/*!
+ * A base class for an updater that uses an external library.
+ * This class contains basic functions to control the updater.
+ *
+ * To implement the updater on a new platform, create a new class that inherits from this class and
+ * implement the pure virtual functions.
+ *
+ * The initializer of the new class should have the side effect of starting the automatic updater. That is,
+ * once the class is initialized, the program should automatically check for updates if necessary.
+ */
+class ExternalUpdater : public QObject
+{
+    Q_OBJECT
+
+public:
+    /*!
+     * Check for updates manually, showing the user a progress bar and an alert if no updates are found.
+     */
+    virtual void checkForUpdates() = 0;
+
+    /*!
+     * Indicates whether or not to check for updates automatically.
+     */
+    virtual bool getAutomaticallyChecksForUpdates() = 0;
+
+    /*!
+     * Indicates the current automatic update check interval in seconds.
+     */
+    virtual double getUpdateCheckInterval() = 0;
+
+    /*!
+     * Indicates whether or not beta updates should be checked for in addition to regular releases.
+     */
+    virtual bool getBetaAllowed() = 0;
+
+    /*!
+     * Set whether or not to check for updates automatically.
+     */
+    virtual void setAutomaticallyChecksForUpdates(bool check) = 0;
+
+    /*!
+     * Set the current automatic update check interval in seconds.
+     */
+    virtual void setUpdateCheckInterval(double seconds) = 0;
+
+    /*!
+     * Set whether or not beta updates should be checked for in addition to regular releases.
+     */
+    virtual void setBetaAllowed(bool allowed) = 0;
+
+signals:
+    /*!
+     * Emits whenever the user's ability to check for updates changes.
+     *
+     * As per Sparkle documentation, "An update check can be made by the user when an update session isn’t in progress,
+     * or when an update or its progress is being shown to the user. A user cannot check for updates when data (such
+     * as the feed or an update) is still being downloaded automatically in the background.
+     *
+     * This property is suitable to use for menu item validation for seeing if checkForUpdates can be invoked."
+     */
+    void canCheckForUpdatesChanged(bool canCheck);
+};
+
+#endif //LAUNCHER_EXTERNALUPDATER_H