Remove all dependency on the browser window's selection

This patch removes all access to the browser's underlying focus. This is an important step towards supporting touch events and non-english input method editors. In order to access the browser's built-in abilities, the window's actual focus and selection must be allowed to remain in an editable text field somewhere. The first step towards enabling this is breaking the final dependencies on the browser selection reflecting the WebODF selection.

Note, this has been tested to work with IE11, Chrome & Firefox, but I don't have easy access to earlier versions of IE to ensure correct behaviour.

Using the caretPositionFromPoint logic seems to also fix issue #30.