Schematic Editor: add function to disconnect/reconnect wire about librepcb

[Schorsch99]

What I meant was to disconnect the wire with the select tool. The procedure could be something like this:

• click on a wire or segment to select it > the end points of the segment are highlighted
• now you can click and drag on the endpoints to move them to another component or pin

[ubruhin]

I'm not sure if I understand correctly - are you talking about disconnecting wires in the wire tool, i.e. to immediately undo a connection made by mistake, or do you like to disconnect wires in general, i.e. in the "select" tool? For the former, this sounds similar to what has been requested in #606 and #1028, just for the schematic editor instead of the board editor. So I think it should be implemented similarly (e.g. same keyboard shortcut).