cleanup: Use more template strings

xgettext got better at recognizing template strings, so we can
replace more string concatenations. Alas xgettext is still buggy
(surprise, regular expressions are hard), so there are still a
handful of holdouts that prevent us from making a complete switch.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/792

js/ui/magnifierDBus.js

@@ -137,7 +137,7 @@ var ShellMagnifier = class ShellMagnifier {
             }
             if (!found) {
                 // Got a ZoomRegion with no DBus proxy, make one.
-                let newPath =  ZOOM_SERVICE_PATH + '/zoomer' + _zoomRegionInstanceCount;
+                let newPath = `${ZOOM_SERVICE_PATH}/zoomer${_zoomRegionInstanceCount}`;
                 _zoomRegionInstanceCount++;
                 let zoomRegionProxy = new ShellMagnifierZoomRegion(newPath, aZoomRegion);
                 let proxyAndZoomer = {};