tp_base_connection_dup_contact_attributes: take a TpHandleSet

It was either that or a GVariant. I took this path as
tp_base_contact_list_dup_contacts() returns a TpHandleSet as well.

https://bugs.freedesktop.org/show_bug.cgi?id=77773
add TpMutableContactList typedef and casting macro
pass a TpMutableContactList to vfunc functions

https://bugs.freedesktop.org/show_bug.cgi?id=77772
pass a TpMutableContactList to methods
rename TpMutableContactList methods
pass a TpMutableContactGroupList to methods
rename TpMutableContactGroupList methods
pass a TpMutableContactGroupList to get_group_storage() vfunc
rename @normalized_group arg to @group

For some reason the gir scanner doesn't seem to like this name?!
introspect base-contact-list
dbus-tube-channel: no need to set result twice
fix signature of TpProxy::invalidated callbacks
tp_handle_ensure: annotate @context with (nullable)
tp_simple_async_report_success_in_idle: annotate @source_tag with (nullable)
tp_handle_ensure: add some pre-conditions
debug: @flags_string and @ignored are (nullable)
tp_handles_are_valid_variant: add some pre-condition checks
dbus.c: add some safe pre-condition checks
tp_simple_async_report_success_in_idle: @user_data is nullable
dbus-tube offerer example: fix type error