Add wire (node) drawing to schema mode

Add wire (node) drawing to schema mode

Using the "c" key while in schema mode will start the drawing.
When it is started over an existing node, the node is extended, else new
node is created.
When ending on a node, the drawn node and the end node are merged
together.
To exit the mode use any other key than the movement keys (best use
"c"?).

To choose the correct symbol for a segment, the t_node_segment had to be
changed, and with that some functions (unrelated to wire drawing) had to
be changed also.