Ensure expand to word and paragraph boundaries keep selection direction

SessionController had some internal confusion about whether to work in
selection or range+bool. This cleans these up to ensure the direction is
properly preserved throughought.
Constrain anchor and focus nodes to the containing paragraph
Restore word & paragraph selection behaviour (double+triple click)

No longer using the browser's selection model means all the browser
behaviours need to be re-implemented in WebODF.