Changes

# <s>Fix accurateness of mouse positioning on unlock() (should be the same point as when lock() was called). Currently works, but is offset. See nsEventStateManager::SetMouseLock. </s> JSilver999

# <s>Restructure Lock method to do most of its operations in a separate thread.</s> humph

# <s>Do we need to add a user pref to enable/disable mouse lock? Nice to have, not blocking.</s> northwind, mjschranz