fix draw_idle reference in  NavigationToolbar2

NavigationToolbar2 sub-classes do not (all?) have a draw_idle method (strictly speaking, the QT one does not).  I assume this should have bounce strait to the canvas, not through the `NavigationToolbar2.draw()` method.

Change introduced in 25ff2f70764b71c7126929e0ef08e7b18c42a0d4
BF: guard against broken PyQt import

We may have picked up a broken PyQt that correctly imported, for example
if we have imported a Python 3.3 PyQt from Python 3.2; check we do have
the attribute we need for getting PyQt versions.
BUG FIX

Fix the incorrectly named Qt.pyqtSignature and Qt.pyqtProperty functions introduced in a previous merge
Stop testing for QString, fixes AttributeError in PySide
Merge pull request #2328 from mspacek/qtapi

Fix PySide compatibility
make all optional backends respect the cfg file
except AttributeError in module gi during checking for gtk3 backends
Fix wrong syntax for assert

The assert statement doesn't take parentheses, see
http://docs.python.org/3.2/reference/simple_stmts.html#assert
http://docs.python.org/2/reference/simple_stmts.html#grammar-token-assert_stmt

I found this because python gave me the following
```SyntaxWarning: assertion is always true, perhaps remove parentheses?```
Remove disabled code.
make sure we only perform absolute imports on loading a backend

Fixes #2309
Merge pull request #2333 from andreabedini/patch-3

Fix wrong syntax for assert
Merge pull request #2308 from dmcdougall/v1.2.x

Make pyplot.bar color kwarg less ambiguous
Update pyplot_formatstr.py
Update pyplot_mathtext.py
Update pyplot_text.py
Update pyplot_three.py
Update pyplot_two_subplots.py
Fix #2338: pyparsing in 1.5.7's <<= operator doesn't return self, so we need to monkey patch it, too
Merge pull request #2344 from anshbansal/master

Fixed the issue of pyplot tutorial missing the show() command
Merge pull request #2344 from anshbansal/master

Fixed the issue of pyplot tutorial missing the show() command