Theming: Add an option to auto add child controls types

[me2beats]

If a beginner tries to customize a theme for a FileDialog, he first needs to know what nodes the control (FileDialog) consists of.
In particular, it contains a Tree node (this may not be obvious). This means that the Tree type probably will be tweaked by the user as well.

So I think when the user adds the FileDialog type, a window should appear that will offer to also add child types (Tree, Button, LineEdit etc), and the user could add all or just some of them.