window: Use GtkRevealer for some buttons

The Requests button now slides out from the add button (+) and the
Accept and Discard buttons fade in and out.
tox: Fix property name

Change "tox-id" to "tox-id-hex" to make clear it is not the binary id
but a hex string.
New widget: request widget

This is now shown instead of the usual chat widget when clicking on a
request contact. This introduces a clean visual separation of requests
and contacts. The requests widget shows the Tox ID of the requesting
contact and the message sent along with the request.
request widget: Some minor tweaks
window: Track the active contact explicitly

The old version of neuland_window_get_active_contact() was bloated and
also caused a crash when trying to get the contact from the new request
widget.