Add a select mode

When typing 'S', you enter the select mode and can
 - select and delete nodes and arcs
 - move nodes and arcs labels

When typing 'A', you enter the path mode, or if the mode
was already path mode, you enter the star mode, in which
you can, as it was implemented before:
- select and delete nodes and arcs
- add arcs between one or two nodes
- move arcs
- move nodes and arcs labels

The select mode is a safe mode.
Delete the unecessary colon symbols of labels

When a node or arc label only contain the id, do not print
the colon symbol.