[server] Send key events to all wl_keyboard resources of a client

Summary:
This is what Weston does. With this change clients can create multiple
wl_keyboard instances and thus get events reported to all of them. This
will be needed to e.g. support KModifierKeyInfo on Wayland.

Similar changes are probably also needed for pointer and touch.

Test Plan:
Auto test for seat still passes. A custom change to kscreenlocker
is able to report whether caps lock is on with this change.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D2963