Fix: missing system logo and app icons on Guix System (#616)

* Fix for Guix logo not being shown

* Fixed icons not being shown in Workspace Switcher. Also added a DesktopService with a function to get the icon path

* Fixed some icons not being shown + Icons in app drawer

* Fixed icons not appearing in Spotlight

* Adapted missing icons in app launcher/spotlight

* Removed (now) useless change

Widgets/AppLauncherListDelegate.qml

@@ -45,7 +45,7 @@ Rectangle {
             width: root.iconSize
             height: root.iconSize
             anchors.verticalCenter: parent.verticalCenter
-            iconValue: model.icon || ""
+            iconValue: model.icon && model.icon !== "" ? model.icon : model.startupClass
             iconSize: root.iconSize
             fallbackText: (model.name && model.name.length > 0) ? model.name.charAt(0).toUpperCase() : "A"
             iconMargins: root.iconMargins