[libtaskmanager] Consider _GTK_APPLICATION_ID for matching desktop files

Despite our best effors we currently do not match Gedit windows to their desktop file. It manages to evade all of our heuristics.

There is a reliable way to map it and other GTK apps though, the _GTK_APPLICATION_ID window property, which is more or less the same as our desktopFileName property

libtaskmanager/xwindowtasksmodel.cpp

@@ -32,8 +32,8 @@ namespace TaskManager
 {
 static const NET::Properties windowInfoFlags =
     NET::WMState | NET::XAWMState | NET::WMDesktop | NET::WMVisibleName | NET::WMGeometry | NET::WMFrameExtents | NET::WMWindowType | NET::WMPid;
-static const NET::Properties2 windowInfoFlags2 =
-    NET::WM2DesktopFileName | NET::WM2Activities | NET::WM2WindowClass | NET::WM2AllowedActions | NET::WM2AppMenuObjectPath | NET::WM2AppMenuServiceName;
+static const NET::Properties2 windowInfoFlags2 = NET::WM2DesktopFileName | NET::WM2Activities | NET::WM2WindowClass | NET::WM2AllowedActions
+    | NET::WM2AppMenuObjectPath | NET::WM2AppMenuServiceName | NET::WM2GTKApplicationId;
 
 class Q_DECL_HIDDEN XWindowTasksModel::Private
 {
@@ -494,6 +494,10 @@ QUrl XWindowTasksModel::Private::windowUrl(WId window)
 
     QString desktopFile = QString::fromUtf8(info->desktopFileName());
 
+    if (desktopFile.isEmpty()) {
+        desktopFile = QString::fromUtf8(info->gtkApplicationId());
+    }
+
     if (!desktopFile.isEmpty()) {
         KService::Ptr service = KService::serviceByStorageId(desktopFile);