NVDAHelper IME support: only call hasValidIMEContext for specific IME window messages, rather than for all window messages. Fixes #2909

This stops ImmLockIMC from being called for window messages such as wm_keydown which was causing some corruption of typed characters in certain situations such as when typing in NVDA's process with Korean or Japanese when it was the default IME.
Move last bug fix in changes file up to 2013.1.1
Merge branch 'release-2013.1' of git://git.nvaccess.org/nvda into release-2013.1jp