contact row: Fix indentation
Small updates for contact rows

The gtk_container_foreach functions for the list boxes now have to use
the rows directly and not get the child first.
window: Use row separators for request

contacts_list_box already had them, and requests_list_box can use the
same list_box_header_func.
window: Remove unused function
window: Fix copy and paste error
contact row: Add "selected" property

This mirrors the state of the check button, using a bidirectional
binding.
contact row: Add API for the "selected" property

Add a toggle function for the property. Also select/unselect rows when
activating them (clicking anywhere, not just the check button) when in
selection mode and unselect all rows when leaving the selection mode.

Note: The GTK_SELECTION_MULTIPLE selection mode for GtkListBox is
currently not usable here, as it 1) is currently broken for pointer
devices and 2) doesn't offer check boxes by default anyway. So we keep
the GTK_SELECTION_SINGLE selection mode and implement multiple selection
with the use of the "selected" property.
window: Implement deleting selection

Multiple contacts can be selected and deleted at once now.