Fix a bug in the cached cursor pos, and simplify selection handling

The documentation for setComposingRegion states explicitly
that it does not move the cursor. This is just a bug.
This does not have any ill effects right now, but it will have
in later changes if not fixed.

As for the selection handling, the specific test that this code
removes used to serve a purpose, but it does not any more because
the code using the value has been much sanitized. Now the variable
can just take the obvious value, and become so self-explanatory
that the comments are unnecessary.

Change-Id: I548d899b38776bd3ab5f5361aab0d89d98f12e73