data: Add key press/release events

While we have "button-{pressed,released}" events there's often the
desire to control sound for key presses of e.g. on screen keyboards too.
Add separate events for that.

This allows us to e.g. add a particular sound for OSK key presses and
OSKs can send this with an `important` hint so they're also usable
in silent mode.

Closes: https://gitlab.freedesktop.org/agx/feedbackd/-/issues/85
Signed-off-by: Guido Günther <agx@sigxcpu.org>

doc/Event-naming-spec-0.0.0.md

@@ -109,6 +109,8 @@ These events are meant to be used by system components.
 - *battery-caution*: The event used when the battery is nearing exhaustion (below 40%, for example).
 - *battery-full*: The event used when the battery is fully loaded up.
 - *device-added*: The event used when a device has become available to the desktop, i.e. due to USB plugging.
+- *key-pressed*: The event used when a key on e.g. an on screen keyboard is pressed.
+- *key-released*: The event used when a key on e.g. an on screen keyboard is released.
 - *message-missed-notification*: The event used when a notification was shown but not seen by the user. (*deprecated*)
 - *message-new-cellbroadcast*: The event used when a new cell broadcast message is received.
 - *notification-missed-generic*: The event used when a generic notification was