moved filechooser.[ch]pp to display/
Moved sdl_utils.[ch]pp to display.

Began refactoring of sdl_utils to use opencv image manipulation
functions instead of custom Wesnoth ones. Purpose is to both enable
improved performance and to simplify codebase. Changes include updated
to SConstruct to build with opencv.

Currently have replace scaling functions and rotation functions with
wrappers around opencv. Marked old functions as deprecated in docstrings
and with __attribute__.

Also refactoring to throw exceptions instead of just returning NULL.
This seems to be a better way to use c++ for long-term maintainability.

Finally, added std::cerr << warnings about pointless calls. These need
to be changed to use the logging framework.

Everything builds and plays.