Child Window Height / Width

**Is your feature request related to a problem? Please describe.**
The `dpg.child_winodw` takes the whole space if you don't provide a height / width parameter.

**Describe the solution you'd like**
It would be great to have a parameter like height_of_content and width_of_content like the extend on the table. With that parameter the child_window can be used anywhere also as dynamic item.