BUG: Fix linux clipboard QApplication() creation
 - [x] closes #14372

A Qt application cannot instantiate multiple `QApplication` instances,
so we create a new `QApplication` only when the global
`QApplication.instance()` is None.

Failing sample (gtk should not be installed for pandas to use qt clipboard):

```
from PyQt4.QtGui import QApplication
myapp = QApplication([])
from pandas.util.clipboard import clipboard_get  # <--- ERROR

  File "prefix/lib/python2.7/site-packages/pandas/util/clipboard.py", line 164, in <module>

    app = qt4.QtGui.QApplication([])

RuntimeError: A QApplication instance already exists.

```
this should be addressed upstream

pls file a report with pyoerclip
if it's accepted there then we can merge  ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14815?src=pr) is 85.27% (diff: 0.00%)
> Merging [#14815](https://codecov.io/gh/pandas-dev/pandas/pull/14815?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

```diff
@@             master     #14815   diff @@
==========================================
  Files           144        144          
  Lines         50968      50968          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43466      43465     -1   
- Misses         7502       7503     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [2466ecb...40d70f9](https://codecov.io/gh/pandas-dev/pandas/compare/2466ecbb717d8cdfd30cc20d5d22e5e095d9a14d...40d70f9c185befc9288f41fb51c2567013fa880f?src=pr) can you add a whatsnew note (0.20.0) thanks!