Proof of concept implementation for delete-surrounding-text.

The QInput method for committing text can be used to replace a region of
text by an empty string, effectively allowing us to delete text.

BUG=http://code.google.com/p/ibus/issues/detail?id=1642

Review URL: https://codereview.appspot.com/13361046
Patch from Benoit Pierre <benoit.pierre@gmail.com>.