Port the panel-plugin to Gtk3

• This is a functional branch of xfce4-notes panel-plugin ported to gtk3 (a branch I had started locally since ages).
• xfce4-notes itself already support gtk3 (already on master), this is "only" the panel plugin part.
• It has tons of deprecation warnings, but from the few tests I did, it "works".
• In maintainer mode (git), you need valac, and libxfce4util, xfconf and xfce4-panel compiled with vala support. It does not need xfce4-vala package anymore.
• The release tarball (generated with make distcheck) do not need valac to build, as C files are provided

If you are interested in helping finishing the work, you are welcome !