API: Cursor style property about revery

[bryphe]

For example, even though its great we have some 'clickability' in #138 - the experience is not optimal:

When the mouse is over the 'Click Me' button, we should change from the pointer to a hand to indicate 'clickability. (There are also broader accessibility issues we'll need to work on here)

[OhadRau]

I've gotten some work done here (mainly getting the style API implemented) but I'm kinda unsure of how to decide which node's cursor to take. I think a simple (but maybe hacky) way of deciding this is to choose the inner-most node as defined by "depth" from the root node. However, this makes the assumption that child nodes are always strictly on top of the parent node. Another option is picking the nearest edge/corner of a bounding box, but this is more computationally intensive and can also fail.

Edge case examples:

• Breaks for corner and sometimes edge
• Breaks for depth

That said, all these will probably need to be replaced with some kind of raycasting algorithm to determine who drew the top-most visible pixel if we want to support non-rectangular bounding boxes so I don't think the choice will be very impactful long-term.

Apart from that, I'm still getting a weird error when trying to compile with both reason-fontkit + reason-glfw 3.2.1008 at the same time, so all this is kind of on hold until I figure out how to fix that.

[OhadRau]

Should be fixed now with #168