Present this after having processed the `GdkToplevelLayout` rules.
If the toplevel was previously not showing, it will be showed, otherwise it will change layout according to layout.
GDK may emit the [[email protected]:GdkToplevel:compute-size] signal to let the user of this toplevel compute the
preferred size of the toplevel surface.
Presenting is asynchronous and the specified layout parameters are not guaranteed to be respected.
| this |
the `GdkToplevel` to show |
| layout |
the `GdkToplevelLayout` object used to layout |