input-settings: Wire up middle-emulation

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

GNOME/mutter!256
5 jobs for !256 with middle-emulation in 5 minutes and 16 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #399624
check-commit-log

00:00:21

 
  Build
passed #399625
build-mutter

00:02:29

passed #399626
build-without-native-backend

00:02:32

 
  Test
passed #399628
can-build-gnome-shell

00:01:29

failed #399627
test-mutter

00:02:21

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

84/92 mutter:core+mutter/stacking / closed-transient-no-input-parents-queued-default-focus-destroyed FAIL 0.67 s (exit status 133 or signal 5 SIGTRAP)

--- command ---
G_TEST_BUILDDIR='/builds/gaul/mutter/build' G_TEST_SRCDIR='/builds/gaul/mutter/src' MUTTER_TEST_PLUGIN_PATH='/builds/gaul/mutter/build/src/compositor/plugins/libdefault.so' catchsegv /builds/gaul/mutter/build/src/tests/mutter-test-runner /builds/gaul/mutter/build/../src/tests/stacking/closed-transient-no-input-parents-queued-default-focus-destroyed.metatest
-------

ERROR: Job failed: exit code 1