Refactoring of IE driver native events interaction code.

This commit removes the dependency on shared native event code with
Firefox. Since Firefox no longer supports native events, it makes no
sense to have common code refactored for only one consumer, so this
commit moves the Windows-only SendMessage code directly into the IE
driver. This is a first step toward deleting the webdriver-interactions
directory. While extreme care has been taken to ensure this refactoring
causes no regressions, there is a chance something will still break.
This particular refactor should also be treated as an intermediate
step, as the ported code contains some redundancies to what is already
present in the driver. Future refactoring will take place to remove the
redundancies and better integrate the code.