Add SVG multi export

Add a second SVG export option that allows for exporting
a series of icons of various size in one go. A dialog is shown
that allows for entering a file name pattern with place holders
and a list of sizes.

Change-Id: Ic644a9d402aa44af5899c29cf83051fdd7bba3d1
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Diana de Sousa <diana.desousa@theqtcompany.com>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/imageviewer/imageviewer.h

@@ -55,6 +55,7 @@ public:
     IEditor *duplicate() override;
 
     void exportImage();
+    void exportMultiImages();
     void imageSizeUpdated(const QSize &size);
     void scaleFactorUpdate(qreal factor);