window: improve swallowing functionality

cleanups, fixes, etc.

ref #6095

src/desktop/Window.hpp

@@ -450,6 +450,7 @@ class CWindow {
     std::string              fetchTitle();
     std::string              fetchClass();
     void                     warpCursor();
+    PHLWINDOW                getSwallower();
 
     // listeners
     void onAck(uint32_t serial);