Add a drag-and-drop API

[zbaylin]

One thing that would be really nice to have in Revery and Onivim2 would be a drag-and-drop API for system as well as internal elements.

The bindings for the system elements (text & files) exist in reason-sdl2 (revery-ui/reason-sdl2#67) as of today, so all that's required there is plumbing that through the Revery event system!

I think this would closely mirror Revery's Mouse events (src/UI/Mouse.re), because after all you have to both drag and drop with your mouse 🐭.

I wanted to make this issue to centralize the progress made on it, because I couldn't find one that existed already.

Some helpful links:

• https://developer.mozilla.org/en-US/docs/Web/API/HTML_Drag_and_Drop_API
• https://wiki.libsdl.org/SDL_DropEvent

[kyldvs]

FYI: This is how I implemented drag & drop in my Revery app a while ago, not sure if it's helpful:

• The core library: https://github.com/kyldvs/chesster/blob/master/src/drag/Drag.re
• Using it: https://github.com/kyldvs/chesster/blob/master/src/ui/ChessBoard.re#L318-L325
• Dummy element to render the dragged piece over things: https://github.com/kyldvs/chesster/blob/master/src/ui/MainView.re#L37