This commit introduces the logic to display ROOT graphics when
using Python interactively. For that purpose, hooks are installed
to process the GUI events and to update the display.

Note that the threading approach is not used anymore: no thread
is explicitly created to process the events. The code based on
hooks should work on all the platforms, but it needs to be
tested. For the same reason, some code that handled the
threading case on Windows has been removed for now, and will be
restored only if necessary.

Most of the code has been picked from several parts of the
current ROOT.py and put together in the configuration of the
PyROOT TApplication.

The activation of the graphics is triggered only if ROOT is not
in batch mode. Such mode can be set by the user right after
importing ROOT.