xfce4-datetime-plugin: Initial add git (0.6.1)

Version 0.6.1 was release about 3 years ago. The patch to migrate from
libxfcegui4->libxfce4ui hopefully shall be applied mainline [1]. It would have
caused extra efforts to base the patch on 0.6.1 release.

[1] https://bugzilla.xfce.org/show_bug.cgi?id=8064



Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Andreas Müller <schnitzeltony@gmx.de>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>