Wire up middle-emulation to libinput

This allows emulating middle click via simultaneous left and right
click.  Fixes #238.

GNOME/mutter!256
4 jobs for !256 with middle-emulation in 6 minutes and 34 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #326027
check-commit-log

00:00:22

 
  Build
passed #326028
build-mutter

00:04:04

 
  Test
passed #326030
can-build-gnome-shell

00:01:19

failed #326029
test-mutter

00:02:07

 
Name Stage Failure
failed
test-mutter Test
-------

87/88 mutter:core+mutter/unit / normal FAIL 0.57 s (exit status 133 or signal 5 SIGTRAP)

--- command ---
MUTTER_TEST_PLUGIN_PATH='/builds/gaul/mutter/build/src/compositor/plugins/libdefault.so' G_TEST_BUILDDIR='/builds/gaul/mutter/build' G_TEST_SRCDIR='/builds/gaul/mutter/src' catchsegv /builds/gaul/mutter/build/src/tests/mutter-test-unit-tests
-------

ERROR: Job failed: exit code 1