Working #256/Fixed #255: Opacity/ledger lines

Ledger lines now disappear when you press “h” hotkey. Temporarily disabled the opacity sliders in order to move on to other things but we will revisit the issue. Additionally updated the shift-clik alerts since they are able to shift click at least 2 neumes.

Fixed #252: Opacity now works

the opacity sliders for glyph opacity and background image are now functional. They stopped working after zoom was implemented but they now work.