Gets the child in the (x, y) position.
Both x and y are assumed to be relative to the origin of this.
| this |
a `GtkFlowBox` |
| x |
the x coordinate of the child |
| y |
the y coordinate of the child |
|
the child widget, which will always be a `GtkFlowBoxChild` or null in case no child widget exists for the given x and y coordinates. |