room_history: re-enable kinetic scrolling after loading more

Previously kinetic scrolling was disabled while loading more
messages and was never re-enabled. This could lead to rooms
not scrolling with touch gestures.

Now after we add widgets to the top, we re-enable kinetic
scrolling for our ScrolledWindow.

Fixes #458
4 jobs from master in 24 minutes and 40 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Check
passed #245323
check

00:00:23

 
  Test
passed #245324
flatpak

00:17:06

 
  Deploy
passed #245325
pages

00:07:10

passed pages:deploy

00:00:02