Merge pull request #8 from timholy/imageview

Enhancements en route to supporting ImageView
Add constants for special keys
Add get_pointer (returns current pointer location within a window)

GdkEventKey does not contain the current pointer location, so this is
needed for handling keypress events whose action depends on the
current pointer position.
Add connect() to simplify passing data to callback functions

This also
- fixes a library-specification problem in some signal-handling functions
- adds tests on signal callbacks