AddEndpointFromPendingConnection removes PendingConnection from Network

[FabianNitsche]

I had the problem, that when I combine the AddNodeFromPendingConnection context menu with the AddEndpointDropPanel, that the connection was correctly created for the GroupIoNode, but after that the context menu poped up. I had no way of telling that AddEndpointDropPanel has already taken care of the PendingConnection.

With this PR this is fixed, by removing the pending connection from the NetworkViewModel.