Add virtual hardware keyboard

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As a games developer
I would like to be able to access a map of virtual keys
So that I can:

• Quickly detect whether a specified key is down or up
• Detect more than one key press at a time
  And for those keystates to be managed automatically by the MOS

Acceptance Criteria:

• Keystates to be stored in a compact format (bit-packed)
• API calls for:
  • Getting the address of the keymap structure
  • Detecting whether a given key is pressed or not

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Note that this currently uses the key values in this table for compatibility.

https://beebwiki.mdfs.net/Negative_INKEY_numbers

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This was added in 1.04 RC2