Mark GTK and Qt clients as GUI programs on Windows (detach from console)

cmake/TrMacros.cmake

@@ -80,3 +80,11 @@ macro(tr_add_external_auto_library ID LIBNAME)
         set(${ID}_LIBRARIES ${${ID}_LIBRARY})
     endif()
 endmacro()
+
+function(tr_append_target_property TGT PROP VAL)
+    get_target_property(OVAL ${TGT} ${PROP})
+    if(OVAL)
+        set(VAL "${OVAL} ${VAL}")
+    endif()
+    set_target_properties(${TGT} PROPERTIES ${PROP} "${VAL}")
+endfunction()