xfce4-settings: update to 4.12.0

* Source is taken from external git repo as before due to problems patching
  binary files (icons). The revision is 4.12 release + Date/Time setter which
  was not accepted upstream.
* Touchscreen/invisible mouse pointer patches do not apply anymore and it is
  not a matter of simple rebasing due to new libinput support. Patches were
  kept because reimplemementation is in progress. This - together with
  libinput support - will be sent in a follow up patch.
* git repo was moved to github

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>