Add more event support

[jaredjj3]

This PR fixes #227 by adding a basic event system. It adds an overlay element, which makes the event behavior consistent between SVG and canvas. Without this, SVG elements would "steal" touchend and touchcancel. I decided to leave the event system very bare and simple for now. I'll try to revisit before 1.0.0.

[jaredjj3]

It offers a slight advantage when an accidental is rendered, but the same could be achieve with boxes. Take a look at how the circle is off-center for notes with accidentals:

Vexflow includes the accidentals, flags and other modifiers in the bounding box. Of course bounding boxes are sometimes too big. See third note.

@rvilarl let's continue the conversation from #230 (comment) here.

I'm not getting the same result as the test. I'm using vexflow.StaveNote.getBoundingBox.

In the tests, I saw something dealing with ModifierContext that I'm not doing in vexml explicitly. I logged vexflow.StaveNote.getModifierContext and got this:

I suspect that this ModifierContext is incorrect. What's the correct way to use this so that the bounding box includes the modifiers?

[rvilarl]

To me the ModifierContext looks good: there is one Accidental and one StaveNote as expected.
What version of VexFlow are you using now? Are you adding the Modifiers to the notes (with addModifier)?

[jaredjj3]

I'm using vxflw-early-access 5.0.0-alpha.11 and I'm using vexflow.StaveNote.addModifier (code).

[rvilarl]

I'm using vxflw-early-access 5.0.0-alpha.11 and I'm using vexflow.StaveNote.addModifier (code).

@jaredjj3 please try with vexflow 5.0.0-alpha.4 we do not release now in vxflw-early-access.

[rvilarl]

@jaredjj3 this is why I said that I had been working in BoundigBox. I resolved some problems but I expect more bugs. Let me know if you come across them.

[jaredjj3]

@rvilarl will do, thanks for improving it!