Update checking feature (#2648)

* Check on startup (toggleable setting) and manually
* Option to check for pre-releases (eg, 2.4.0-beta1)
* Only included if WITH_XC_NETWORKING is enabled

src/gui/MainWindow.h

@@ -83,6 +83,9 @@ private slots:
     void setMenuActionState(DatabaseWidget::Mode mode = DatabaseWidget::Mode::None);
     void updateWindowTitle();
     void showAboutDialog();
+    void showUpdateCheckStartup();
+    void showUpdateCheckDialog();
+    void hasUpdateAvailable(bool hasUpdate, const QString& version, bool isManuallyRequested);
     void openDonateUrl();
     void openBugReportUrl();
     void switchToDatabases();