window: Harmonize function names for actions
window: Never automatically activate a contact

We really shouldn't open a random chat view when starting up.
window: Remember active contact and request

When toggling between contacts and requests, remember which
contact/request was shown before and activate it again.
window: Remove unused variables in function
window: Automatically activate the first request

When switching to requests we actually want the first request to be
activated automatically (if none had activated yet), because requests
are more like a list that you accept or discard one by one.
window: Simplify code for Accept/Discard buttons

Showing and hiding these buttons can be handled centrally in
neuland_window_show_requests_state_changed().
tox: The remove func for contacts now takes a list

This replaces neuland_tox_remove_contact() which could only remove one
contact at a time with neuland_tox_remove_contacts() which takes a
GSList of contacts.