Allow to specify edge metadata

[jonnor]

One usecase is for specifying connection capacities on buffered, for runtimes which support this (currently JavaFBP).

If the UI is specific to this purpose, it may be neccesary to add a capability to the protocol for this feature so we can show-hide it as appropriate.

[bergie]

This should be added to the edge inspector

[jpaulm]

In the pre-NoFlo .fbp notation, connection capacity is specified by a number in brackets after the arrow. This seems to me the logical place to put it. Putting it as metadata associated with the target component actually works for "classical" FBP because of the rule against one output port feeding multiple input ports, but it's still slightly counterintuitive.

Whichever convention we adopt, we need to decide on a syntax, and document it.

[jonnor]

Related: node metadata, #482

[bergie]

An example of an edge metadata property editor is #472