Edit edge without endpoint dragging, and pass label in data

[eymiha]

fixes #2233
fixes #2068
fixes #1756

This adjusts the docs to the actual old mechanism and adds new code to skip the endpoint dragging and go straight to a user-defined edge editor if given.

Adding

{ manipulation: { editEdge: { editWithoutDrag: function } } }

calls the function directly, before any endpoint dragging takes place. If the old mechanism that activates endpoint dragging before the function call is desired, just supply the function directly as the value for edgeEdit.

In addition, the selected edge's label is added to the data passed to the edit function.

Also, when the user edit is cancelled, the edit bar was not being redisplayed. This is also fixed.

[mojoaxel]

@eymiha Could you please add (or improve an existing) example to show this new feature. That way it would be much easier to test.

[eymiha]

I added a quick and dirty example manipulation that edits edge labels via a popup, without dragging first. Edge creation still allows a drag, but the label assignment is included as well.

[mojoaxel]

Thanks! Now I understand! Maybe in the future we can merge the two examples manipulationEditEdgeNoDrag.html and manipulation.html. I think one would be enough.

[mojoaxel]

@eymiha Thanks very much for this nice improvement!