Added value clamping for stereo hint.
Fixed dead keys in X11.

The library will now try to create an X input context to handle dead
keys properly on international keyboards.  This makes it possible to
enter for example an e with accent grave on a German keyboard without
further efforts.  A fallback mechanism is provided in case the client
does not support X input method / context creation.  In that case, the
library will behave as it did before.
Cleanup of basic XIM support.

Shortened and simplified the code.  Removed reporting of XIM sentinel
key press event.  Added credit.  Updated changelog.

Closes #151.
Documentation work.