ExtensionManager: Implement Extension details design

This implements the design for the "right side" of the extension
manager.

The introduced "header" shows the extension icon in a slightly bigger
variant. The "Install..." button that starts downloading and installing
of a plugin moved to the newly desigend "header".

The previous HTML based prototype has been split up into separate items
in order to achieve specialized sections like the images and tags.

Images are loaded via TaskTree and displayed as static image or as
animation.

Change-Id: Ifaf4a46c0a4789e77e76f9a44c8a15ee74c5e8df
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

src/plugins/extensionmanager/extensionmanagerwidget.h

@@ -14,6 +14,7 @@ public:
 private:
     void updateView(const QModelIndex &current);
     void fetchAndInstallPlugin(const QUrl &url);
+    void fetchAndDisplayImage(const QUrl &url);
 
     class ExtensionManagerWidgetPrivate *d = nullptr;
 };