StartConversationEntry: Cancel on empty screen name
UserListsWidget: Don't create lists with empty names
account-list-entry: Remove margin-top

We don't need that anymore since the SettingsDialog isn't using tabs.
main-window: Put avatar_image and compose button into a GtkBox

Needed for the 2 separators in the titlebar to not look different
because of the ".header-bar > GtkBox > .separator.vertical" rule in
Adwaita.
Merge branch 'master' into compose-completion
ProfilePage: more g_message -> g_debug transitions
ComposeTweetWindow: Update completion on cursor change