libei: Update to v1.6.0

[TraceyC77]

Summary
Update libei to the latest version

Changelog:

1.6.0
This release contains a few protocol additions:

ei_device.ready is a request sent by compatible clients after ei_device.done to notify the EIS implementation that the client is done with any device-specific configuration.
No configuration is currently available, it is prep work for future tablet support.
ei_seat.request_device is a request sent by compatible clients to request a device with specific capabilities. The EIS implementation is not required to honor this request. This too is prep work for future tablet support where it will be used to request multiple tablet tools.
a new ei_text interface that provides the ei_text.keysym and ei_text.utf8 requests and events. These allow an emulating client to send keysyms or straight utf8, useful for situations where a keysym needs to be sent independent of the available keymap on the ei_keyboard device.

The protocol and API documentation is available here: https://libinput.pages.freedesktop.org/libei/
In addition to this a bunch of general fixes and the usual spit and polish.

Many thanks to David Redondo, Hongfei Shang, Ian Douglas Scott, Jan Beich, Jason Gerecke and Peter Hutterer.

1.5.0

The protocol remains the same (bar many cleanups, typo fixes and rewordings) but on the C library side we have a few improvements:

previously a slow receiver would get disconnected as soon as the other side encountered a writers' block. This is now handled via a larger queue so disconnections will only happen in extreme cases.
for the socket backend, the new eis_backend_socket_get_client_pid() provides, perhaps unsurprisingly, the client PID
ei_event_ref() and ei_event_unref() are now available to clients in case the event needs to be kept outside the immediate scope. This should still be handled with care.
The handling of ei_pingpong and ei_sync is now that the pong/sync reply is sent when the respective event is unref'd (previously it was sent as soon as the event is retrieved from libei(s)). This allows e.g. an EIS implementation to complete processing of internal state before replying to the client.

And of course a typical grab-bag of fixes all over the place. Many thanks to Axel Karjalainen, David Redondo, Eric Long, Ian Douglas Scott, Jason Gerecke, Jonas Ådahl, and Kacper Piwiński.

Full release notes available here

Test Plan

Built krdp from KDE sources, which requires this library

Checklist

• Package was built and tested against unstable
• This change could gainfully be listed in the weekly sync notes once merged
• I agree to license this contribution and all my previous contributions under the licensing terms in LICENSE.md and have the power and authority to grant those licenses.

[clintre]

I doesn't look like the build copies the license.
Want to go ahead and add the license macro?