Create note field widgets

[Kangaroux]

Closes #24, closes #25

This branch is just for displaying a static note field. You pass it a Chart object and it renders out the taps, holds, and beat lines. As of now the note field cannot be manipulated or interacted with, aside from scrolling

TODO

• [x] 🐞 Fix tap notes being able to stack on top of hold notes
• [x] 🐞 Fix beat lines getting cut off when scrolling due to the vertical margin
• [x] Add hold note assets (note and hold body)
• [x] Split up NoteField. Pull out the logic/config into a separate class and pass that to the chart object widgets instead of the entire Gtk container.
• [x] Prevent scrolling beyond the end of the chart (4 measure buffer after the last note)
• [x] Change Cairo surface/pattern object in HoldNoteBody.cs to be static
• [x] Add any missing event listeners
• [x] Add some UI tests. Doesn't have to be too comprehensive, just high level to make sure everything looks ok
• [x] Add missing comments/documentation (docs needed?)

[Kangaroux]

@monotron For the UI tests I tested the NoteFieldData class but didn't test any widgets. The widgets themselves are pretty logicless, they are just grabbing values from a NoteFieldData object, plus it's easy to see if the widgets are working properly just by using the editor.

If you think there are some low hanging fruit with regards to widget tests let me know.

[emberdex]

If you think there are some low hanging fruit with regards to widget tests let me know.

Nothing that springs to mind immediately, but I will let you know if something comes up!