New panel plugins - Clipman and Appmenu

[Yousuf Philips]

Hi Team,

I've proposed adding the clipman plugin to the default panel and adding the
appmenu plugin for use by the new Cupertino panel layout. If you have any
opinions about this, please reply or leave a comment on github.

https://github.com/Xubuntu/xubuntu-default-settings/issues/14
https://github.com/philipzae/xubuntu-seed/commit/1d412bfd17ddc9a599753b0084fb03a361365831
https://github.com/Xubuntu/xubuntu-default-settings/pull/15

Regards,
Yousuf