wayland: Add wayland client

This patch adds a new client which supports the Wayland input method
protocol.  Note that the support is disabled by default until the
input method protocol becomes official in the Wayland upstream.
Supply --enable-wayland to configure to try it.  Also, you will need
to set the executable path of ibus-wayland under the "input-method"
section of weston.ini, like this:

 [input-method]
 path=.../libexec/ibus-wayland

BUG=Issue#1617

Review URL: https://codereview.appspot.com/11320043
Patch from Jan Arne Petersen <jan.petersen@kdab.com>.