Add focus surface setting of keyboard for copy and paste between clients

Add focus surface setting of keyboard for copy and paste between clients

Summary:
To enable copy and paste between clients,
wayland data device requires to know what surface is focused.
So this revision have make keyboard get focused surface when client is focused or unfocused,
and create data_offer for new focused surface in order to let new focused surface be able to get data from current selection data_source.

Test Plan:
1. Run wayland server
2. Run elementary_test -to entry5 on wayland server.
3. Run elementary_test -to entry5 on wayland server again. (preparing two clients)
4. Copy text on one of clients, and paste to the other.

Reviewers: devilhorns

CC: gwanglim, cedric
Differential Revision: https://phab.enlightenment.org/D1157