make PlaygroundDragElement move search recursively for target

Making my own custom widgets, I ran into an issue with `PlaygroundDragElement` trying to add a complex widget to one if its own children.

Example - add the following to `designer/common.py`:
````
widgets = [
    ...
    ('TabbedPanelTest', 'complex'),
 ]

from kivy.lang import Builder
Builder.load_string('''
<TabbedPanelTest@TabbedPanel>:
	do_default_tab: False
	TabbedPanelItem:
		GridLayout:
''')
````
Create a new project (I tested with the `BoxLayout` template), then add the `TabbedPanelTest` to the project. When you drag the `TabbedPanelTest` over the `BoxLayout` it will be added. Keep dragging the `PlaygroundDragElement` over the nested `GridLayout` and the `TabbedPanelTest` will then be added to its own `GridLayout`, causing the interpreter stack to blow up from an infinite loop attempting to process the graphics instructions.