ENH: Update ExtensionsManager install API to check for update

Add support for the `update` parameter in the
`qSlicerExtensionsManagerModel::installExtensionFromServer` function.

Users can now set the `update` parameter to `true` to schedule the
update of an already installed extension to the latest version available
on the server.

The update will take effect during the next application restart.

If the extension is not already installed, this parameter has no effect.

Co-authored-by: Andras Lasso <lasso@queensu.ca>

Base/QTCore/qSlicerExtensionsManagerModel.cxx

@@ -1714,13 +1714,21 @@ bool qSlicerExtensionsManagerModel::downloadAndInstallExtensionByName(const QStr
 }
 
 //-----------------------------------------------------------------------------
-bool qSlicerExtensionsManagerModel::installExtensionFromServer(const QString& extensionName, bool restart)
+bool qSlicerExtensionsManagerModel::installExtensionFromServer(const QString& extensionName, bool restart, bool update)
 {
   Q_D(qSlicerExtensionsManagerModel);
 
   if (this->isExtensionInstalled(extensionName))
     {
-    // Already installed; nothing to do
+    if (update)
+      {
+      // Ensure extension metadata is retrieved from the server or cache.
+      if (!this->updateExtensionsMetadataFromServer(/* force= */ true, /* waitForCompletion= */ true))
+        {
+        return false;
+        }
+      this->scheduleExtensionForUpdate(extensionName);
+      }
     return true;
     }
 

Base/QTCore/qSlicerExtensionsManagerModel.h

@@ -398,15 +398,22 @@ public slots:
   /// To prevent the application from automatically restarting after the
   /// installation is completed, set the \a restart parameter to false.
   ///
+  /// To schedule the update of the extension and its dependencies to
+  /// the latest version available if already installed, set the \a update
+  /// parameter to true.
+  /// The update will take effect during the next application restart.
+  ///
   /// \param extensionName The name of the extension to be installed.
   /// \param restart Set to false to prevent automatic application restart (default: true).
+  /// \param update Set to true to schedule the extension for an update on the next application restart (default: false).
   ///
   /// \return \c true if the extension is successfully installed or already installed.
   ///
   /// \sa setInteractive
   /// \sa isExtensionInstalled, installExtension, updateExtensionsMetadataFromServer, downloadAndInstallExtensionByName
+  /// \sa scheduleExtensionForUpdate
   /// \sa qSlicerCoreApplication::testAttribute, qSlicerCoreApplication::AA_EnableTesting, qSlicerCoreApplication::restart
-  bool installExtensionFromServer(const QString& extensionName, bool restart = true);
+  bool installExtensionFromServer(const QString& extensionName, bool restart = true, bool update = false);
 
   /// \brief Schedule \a extensionName of uninstall
   /// Tell the application to uninstall \a extensionName when it will restart