Wire up middle-emulation to libinput

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

GNOME/mutter!256
3 jobs for !256 with middle-emulation in 6 minutes and 21 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Review
passed #275542
check-commit-log

00:00:48

 
  Build
passed #275543
build-mutter

00:02:05

 
  Test
failed #275544
test-mutter

00:03:27

 
Name Stage Failure
failed
test-mutter Test

Ok: 24
Expected Fail: 0
Fail: 3
Unexpected Pass: 0
Skipped: 0
Timeout: 0

ERROR: Job failed: exit code 1