panel: Center-align menu arrows

Center alignment looks more balanced and makes it more likely that
the menu opens below the pointer position.

GNOME/gnome-shell#1351
4 jobs for !577 with menu-arrow-alignment in 3 minutes and 57 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #326570
check_commit_log

00:00:23

 
  Build
passed #326571
build

00:02:32

 
  Test
passed #326572
test

00:01:01

passed #326573
test-pot

00:01:01