Minor update - key mapping and $verify

Hi everyone,

I have made some minor updates to the code to achieve 2 things:

• $verify now works
• Arrow keys and Function keys (F1-F4 only) now work

A couple of things on the arrow/function key part of the update:

There are issues with the FabGL terminal code which mean that the keys seem to take time before they first are sent. This makes them not responsive. I've left them in there in the hope that this will be fixed at some point.

There are only 4 function keys implemented - the VT100 terminal only had 4 function keys, so this is reflected in the FabGL/VDP code.

The code works by intercepting the escape sequence sent by the keyboard. This meant some design choices in the programming. I've chosen to do a blocking read after the first escape is received before trying to interpret the rest of the sequence. This has a minor disadadvantage in that if you use the actual escape key, the code pauses to wait for the next character before trying to validate if it is a correct sequence.