fix widget Popup position && the stack order

When i use the popup widget like this:

```python
```

```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)]'}
```