Add up/down hotkeys to Charter

[EliteMasterEric]

I was going through old issues for improvements to make and found this idea: #209. Most of this is implemented (selecting notes, deleting notes) but moving selected notes was not.

This PR performs the following changes:

• Adds up/down hotkeys to move selected notes up/down one step in the charter.
• Adds shift-up/down hotkeys to move selected notes up/down one beat in the charter.
• Renamed the Shift option in the tabs to better indicate it affects all notes.
• Added additional controls to the Help text.
• Fix a crash bug: When placing a note, removing a note, then zooming in or out with CTRL-MWhell, a Null Object Reference occurs.

[Kade-github]

Nice job