Web UI: basic SYSCALL support

[lupino3]

partial SYSCALL support - no File I/O

• fd 1 output via Web UI
• fd 0 input via Web UI

[lupino3]

Standard Input will be more complicated than just having a prompt() call. The Java code runs in a web worker, which does not have access to the window object. I tried using prompt() and alert() and it fails miserably.

[lupino3]

We could add a new StdIn section with a text area, and as soon as a character is added to the text area it's sent to the worker as part of the stdin. Then when SYSCALL 3 happens to read from fd 0, it reads from that stdin buffer. This would remove the need for interactive input being triggered by the web worker.