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 fix-autoscroll-regression in 2 minutes and 44 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #13250
run-appdata

00:00:05

passed #13249
run-tests

00:02:28

 
  Build
passed #13251
run-build

00:00:10