Make the QVTKRenderWindowInteractor more widely compatible.

Fix compatibility issues with recent commit 5316dff1.  It used the new
"import as" syntax that isn't supported on python 2.5 and earlier.
It also used a new signal/slot mechanism that is only available in
PyQt 4.5 and later, even though the old signal/slot mechanism was
still being used elsewhere in the file (mixing old and new ways
of doing the same thing in a single file makes the code hard to read).

The test TestQVTKRenderWindowInteractor.py did not draw anything,
because it didn't connect the renderer to the render window, nor
did it initialize the interactor.  It also used two-space indentation
in some places and four-space indentation in others.

Change-Id: Ie41798554cce2d1ffe5705ec198d191d9e18cb08