Remove Application.Controller::display_main_window_if_ready

It's not actually needed under the new application regime for anything
other than UpgradeDialog, so rework that to not require it and remove
the method and supporting code.
5 jobs for mjog/mutiple-main-windows in 25 minutes and 57 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #502589
fedora

00:05:47

passed #502590
ubuntu

00:12:25

 
  Test
passed #502591
flatpak

00:12:21

 
  Deploy
passed #502592
review

00:00:33

passed #502593
allowed to fail manual
stop_review

00:00:35