Added deleteFromRight/Left functions to some mml elements.
Implemented an EVENT.Beep() method that flashes the mRow

Need to implement:
1) Spilling the content of an mRow into another mRow
2) Give various mml elements the functions that decide whether the mRow needs to be deleted and what to do with the object afterwards
3) Implement code for highlighting of mRow if a delete is pressed under certain conditions
4) Implement the deletion of the highlighted nodes and placemen of the focus afterwards
5) Implement the replacement of highlighted nodes if something is typed
6) Implement the resolution of where the focus should go if an arrow is pressed while some objects are highlighted