ChatControlBase: fix autoscroll on keyboard shortcuts

The issue reported by concerto [1] is a regression since af312830.

[1] https://conference.gajim.org:5281/muc_log/gajim/2019-09-10#df591057-8693-4e76-bbc2-334d839c6adc
3 jobs for master in 2 minutes and 41 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #13253
run-appdata

00:00:05

passed #13252
run-tests

00:02:26

 
  Build
passed #13254
run-build

00:00:10