[Diagram Editor] Improve connection UI/UX

[mxgrey]

There are few ways we could improve the UI/UX of how operations get connected:

• For outputs we could use something like a triangle pointing out of the operation to give the sense that it represents data moving outwards. Regular (final) outputs would be on the bottom of the box pointing down, while stream outputs would be on the right side of the box pointing rightwards.
• The color of stream outputs should be the same as regular outputs since there's no difference in what they can be connected to. Maybe stream outputs can be distinguished with a different kind of icon, e.g. overlapping triangles >> to indicate that multiple outputs may come out of it.
• It might make more sense for Fork Result to have a separate output slot for Ok versus Error. Maybe Error can go on the left or right side while Ok is on the bottom.
• It might be good to use a different output connector icon for operations that allow an arbitrary number of outputs, like Fork Clone and Split. Maybe an icon that's a cluster of circles or something to indicate that many connections are allowed.
• Buffer Access would probably be easier to understand if it had a separate input slot icons for input messages (white) versus buffers (purple). It should also allow multiple input message connections, but at the moment is seems to block any attempt to connect a second input message to it.

[koonpeng]

Multiple handles in the same direction would be tricky for ReactFlow because it doesn't support it natively, though it should be possible to achieve with some css, but it may mess up ReactFlow drawing of edges, snapping etc.

Buffer Access would probably be easier to understand if it had a separate input slot icons for input messages (white) versus buffers (purple). It should also allow multiple input message connections, but at the moment is seems to block any attempt to connect a second input message to it.

It should be allowing multiple inputs already.

[mxgrey]

It should be allowing multiple inputs already.

I tried it again just now and can confirm that it worked. When I tested it previously I might have been trying to connect an output that was already connected to another node, so that was an unrelated error.