cleanup: Prefer template strings

Template strings are much nicer than string concatenation, so use
them where possible; this excludes translatable strings and any
strings containing '/' (until we can depend on gettext >= 0.20[0]).

[0] https://savannah.gnu.org/bugs/?50920

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/612

js/ui/popupMenu.js

@@ -501,7 +501,7 @@ var PopupMenuBase = class {
             let app = Shell.AppSystem.get_default().lookup_app(desktopFile);
 
             if (!app) {
-                log('Settings panel for desktop file ' + desktopFile + ' could not be loaded!');
+                log(`Settings panel for desktop file ${desktopFile} could not be loaded!`);
                 return;
             }