Support multiple terminals

[bartbutenaers]

Multiple terminals can be open at the same moment:

• A single flow can have multiple xterm nodes. But that shouldn't be a problem, since each node gets its own unique node-pty spawned process. But I need to test this.
• Multiple flow editors could open simultaneously the same node, so we get N terminals (client-side) communicating with the same node-pty spawned process backend side. So all terminals will share the same data, which is not what we want. --> Solution: generate a random number in the editprepare and store all node-pty processes in a map on backend side (with key is the unique random number that identifies a specific config screen).

[bartbutenaers]

Implemented