fix widget Popup position && the stack order

When i test the widgets. the last is the popup widget like this:

```python
counter = widgets.IntText(description='Counter:')
popup = widgets.Popup(children=[counter], description='Popup Demo', button_text='Popup Button')
display(popup)
```
but i can't see it. Finally i found it to be positioned in the upper-left. I'm confused. Other widgets are in the below the cell.  so. i think this Popup window must nearby this cell. at least you can see on the screen - no matter how far I have to scroll the screen. or it fixed to below the `maintoolbar`

and. when i first see the popup window. it's "z-index" less than the cell's. i can see the cell's code and can't click the close button

```python
{'commit_hash': u'bb42d9a',
 'commit_source': 'repository',
 'default_encoding': 'UTF-8',
 'ipython_path': '/Users/dongweiming/ipython/IPython',
 'ipython_version': '3.0.0-dev',
 'os_name': 'posix',
 'platform': 'Darwin-13.4.0-x86_64-i386-64bit',
 'sys_executable': '/usr/bin/python',
 'sys_platform': 'darwin',
 'sys_version': '2.7.5 (default, Mar  9 2014, 22:15:05) \n[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)]'}
```