inline and rm Cursor::{write,insertCh}

only createRoot used Cursor::write, and only Cursor::write and the
'cmd' public API call used Cursor::insertCh

this also makes the 'key' and 'text' textarea manager event handlers in
createRoot symmetrical (both call methods on cursor.parent)

I still dislike prepareWrite, but by removing unnecessary indirection
this will make refactoring it away later clearer.