Plover engine hooks

Of the three engine hooks with data for strokes, stroked, send_string and send_backspace, the two send_* hooks will trigger before stroked. Therefore, stroked is used as the trigger to update the strokes data, as by then the number of backspaces and text string Plover emits for the stroke are available. If done the other way around, send_string will not know the present stroke, and the code becomes more complicated with the strokes data always a step behind.

The on_stroked function in the code is the workhorse of the entire editor. It sets properties of the paragraph and modifies them before inserting the text Plover outputs, and also deleting text based on the number of backspaces Plover outputs.