Added a test for empty bar which suppose to fill with rest
Changed event name
Added a function to get the note corresponds to line number
Added helper functions to get note by line
Added click event to music to add note
Changed the way of acquisition of y of lines

o It turns out that y which comes from stave is different than one which
comes from key props. So I follow the same way vexflow does to get that
value.
o Fixed the hot spot issue
Standardized event names