Add feedback during edge creation

[pcdavid]

As mentioned in the corresponding shape:

During the edge creation or reconnection process (the user is dragging a connection line from its source to the wanted target):

• nodes which are known to be incompatible with any kind of edge creation tool starting from the source are temporarily faded;
• when the mouse cursor hovers on a compatible target node, that node is highlighted (using the same style as uses for node DnD);
• nodes (compatible but not hovered) which where not faded by the user keep their style
• nodes (compatible but not hovered) which where faded by the user are temporarily unfaded to indicate they are compatible.

[pcdavid]

Only edge creation will be handled in this issue. Edge reconnection requires more work and will be done later.