UI: Replace GtkPaned with a GtkBox

This is easier as the contacts list should have a fixed width for now
anyway.
welcome widget: Two small fixes

Add suggested-action style class to the add button and make the widget
expand so it centers inside the stack.
window: Add a Requests button

The plan is that this will later only show up when there are pending Tox
friend requests and function as a toggle to show either pending requests
or normal contacts.
contact widget: Reduce width

Reduce the max width of name and status message labels to 20 chars.