ENH: Fullscreen

Closes #387.

I have only changed one example (`text_visual.py`), assuming people are happy with the other changes, I'll change all the other examples.
FYI @almarklein I had to add these in order to prevent really strange testing errors I was encountering because I have PySide and PyQt4 on my system. Briefly, the `sys_info` function uses `has_backend`, which tries to import backends to see if they're available. Doing so made it so PyQt4 was imported, then PySide, and the previous code then left PySide as the working `qt` backend. This then broke the Qt embedding test (sheesh). These changes prevent changing the `qt` mode after it's been set once.