* Fix CV_Asserts with negation of strings

{!"string"} causes some compilers to throw a warning.

The value of the string is not that important -- it's only for printing
the assertion message.

Replace these calls with:

  CV_Error(Error::StsError, "string")

to suppress the warning.

* remove unnecessary 'break' after CV_Error()

fixes based on the feedback: window name prefix is automatically added when it is not there, singleton class returns reference instead of pointer, destructor is private, release function implemented

initial shared pointer implementation for Viz3d, singleton VizAccessor, cv::viz::get function initial implementation