This backend is a event backend that can be enabled for EGL (for now).
It uses udev (gudev) to query input devices on a linux system, listens to
keyboard events from input devices and xkbcommon to translate raw key
codes into key keysyms.
This commit only supports key events, more to follow.
Looking at what the X11 backend does: the unicode value is being
translated to the unicode codepoint of the symbol if possible. Let's do
the same then.
Before that, key events for say KEY_Right (0xff53) had the unicode_value
set to the keysym, which meant "This key event is actually printable and
is Unicode codepoint is 0xff53", which lead to interesting results.
The wayland client code has support for translating raw linux input
device key codes coming from the wayland compositor into key symbols
thanks to libxkbcommon.
A backend directly listening to linux input devices (called evdev, just
like the Xorg one) could use exactly the same code for the translation,
so abstract it a bit in a separate file.
