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.

```
