keyboard: Destroy the keyboard after touchscreen inactivity

In a touch-screen device, as soon as we touch the screen the first time, we
create the keyboard.

However, if the touch screen isn't used for some long time, the keyboard is kept
alive and still around. Although we can suppose that it's not the primary user
device.

So, as soon as the user starts using a non-touch device we start a timeout.
If the user has not used any touch device in the time being, we destroy the
keyboard. And it will be recreated as soon a new touch event happens.

GNOME/gnome-shell!541/
4 jobs for !541 with keyboard-destroy in 5 minutes and 40 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Review
passed #296864
check_commit_log

00:00:39

 
  Build
passed #296865
build

00:04:00

 
  Test
passed #296866
test

00:01:00

passed #296867
test-pot

00:00:58