action-bar: Add an action bar to the conversations list (2-panel)

This moves the actions from the headerbar to the action bar at the
bottom of the conversations list when multiple conversations are
selected. This changes is needed so that the user can still interact
with the conversations when folded.
This also hides the actions from the Headerbar and action bar when
no conversation is selected.
3 jobs for adaptive in 50 minutes and 4 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #935939
fedora

00:04:34

passed #935940
ubuntu

00:07:31

 
  Test
passed #935941
flatpak
flatpak

00:42:32