BUG: Fix linux Qt clipboard QApplication() creation
Fixes #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:

```
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.

```

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14371?src=pr) is 85.26% (diff: 100%)

> Merging [#14371](https://codecov.io/gh/pydata/pandas/pull/14371?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #14371   diff @@
==========================================
  Files           140        140          
  Lines         50632      50632          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43171      43170     -1   
- Misses         7461       7462     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [ee374ee...09eb695](https://codecov.io/gh/pydata/pandas/compare/ee374ee3bc49b82754e8269f69cf8f79d488bd5e...09eb6951db92bea2fd73562e619f92dec4bb7d05?src=pr)
 I don't think any of our clipboard tests actually work on travis, xref https://github.com/pydata/pandas/pull/9693

ideally we can resolve this first.
 our pyperclip has been added quite subtantially in master, so pls re-tests and re-open if needed. Reopened on changed file as https://github.com/pandas-dev/pandas/pull/14815