TweetListEntry: Delete tweet on first try...

... if the user's been using the menu item
MainWindow: Show the left separator again
MainWindow: Don't show app_menu_button if account == null

In this case, we are in "account setup mode" and we don't want to
show anything but the AccountCreateWidget.
Merge branch 'main-widget' of https://github.com/baedert/corebird into main-widget
Show added account in the SettingsDialog
SettingsDialog: Fix AccountListEntry when new account added
MainWidget: Remove old code
MainWindow: Exit early in delete handler if account == NULL
MainWidget: Remove the dummy account here

and not in the MainWindow.
Rename account-create-widget2.ui to account-create-widget.ui
Update the account popover if the details change