support for arrow keys

Mu's keyboard handling is currently a bit of a mess, and this commit might
be a bad idea.

Ideally keyboards would return Unicode. Currently Mu returns single bytes.
Mostly ASCII. No support for international keyboards yet.

ASCII and Unicode have some keyboard scancodes grandfathered in, that don't
really make sense for data transmission. Like backspace and delete. However,
other keyboard scancodes don't have any place in Unicode. Including arrow keys.

So Mu carves out an exception to Unicode for arrow keys. We'll place the
arrow keys in a part of Unicode that is set aside for implementation-defined
behavior (https://en.wikipedia.org/wiki/C0_and_C1_control_codes#C1_controls):

  0x80: left arrow
  0x81: down arrow
  0x82: up arrow
  0x83: right arrow

The order is same as hjkl for mnemonic convenience. I'd _really_ to follow
someone else's cannibalization here. If I find one later, I'll switch to
it.

Applications that blindly assume the keyboard generates Unicode will have
a bad time. Events like backspace, delete and arrow keys are intended to
be processed early and should not be in text.

With a little luck I won't need to modify this convention when I support
international keyboards.

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