window: Fix another welcome widget issue

When there is no active contact when switching to contacts, show the
welcome widget and don't select the contact in the first row.

The idea here is that we don't want any chat to be visible before the
user actually clicked on a contact in the list.
window: Clarify neuland_window_activate* funcs
window: Consistent naming of reject-active action

The associated button was already called header_button_reject, and
'reject' fits better here than 'delete'.
window: Fix buttons in NeulandWindowPrivate

Always use GtkButton and not GtkWidget for consistency.
window: Fix '+' button issue

Don't hide the '+' button ('header_button_create_request') when showing
requests since that shifts the requests button.