[toolchains] Editor resets previous input sync if another input is attached to a block

How to reproduce:

1. Fork a toolchain
2. Create a connection from an output to an already connected input with a different synchronization channel

Results: the old connection will be synchronized with the new channel; a new connection will not be drawn.

One of the two solutions must be adopted:

1. The new connection is not accepted and nothing changes
2. The new connection replaces the old connection