Improve display protocol to avoid requirement for constant polling by controller, thus improving controller battery life

[pachpict]

Comment from D Mielke, re BRLTTY support:

I'd like to plant a couple of ideas for new features into your head:

Add new commands (I suggest with the same values as the old oens but with 0X80
ored) which the device could use to send unsolicited data to the host. For
example, for indicating when the motors are no longer busy, for a change in the
currently rpessed buttons state, etc. That way, the driver wouldn't have to
continually poll and could just go idle and wait.

A enw command that the driver could send to refresh a specific row. I think our
users would really like it, and I bet yours would too. My suggestion is that,
for offline use, you make that be the row key in combination with the square
(refresh) key at the bottom. That's what we've done, although those currently
fall back to the full refresh.

Right now, querying the protocol version yields 1 (which is good enough). I'd
like to request that this number be strictly maintained from now on so that our
driver can know which new features are available for any given device.

Let me know if you'd like a full description of how our driver works. In our
source tree, you'll find the actual code in: Drivers/Braille/Canute/braille.c

[pachpict]

Above comment by @DaveMielke was with reference to his Canute driver in BRLTTY. Now that we are looking into supporting Canute on tablets the issue of constantly having to poll the Canute becomes more acute; it could seriously lower the tablet's battery life if it isn't fixed.