Rename Cursor::setSelection(bool) to selection(bool)

Rename Cursor::setSelection(bool) to selection(bool)

The old name would be confusing wrt setSelection(), which does additional checks.
This one is a pure acessor, and the more complete methods are
* setSelection(), which avoids empty selections
* clearSelection(), which resets anchor, and sets word selection and mark more to false.

Most of the code should use these two instead of selection(bool), but this is for later.