mate-panel: update control, rules and .install files

the change of the applets' libdir is taken from
http://anonscm.debian.org/cgit/pkg-mate/mate-panel.git/commit/debian/rules?id=134557a29ebfb636eacb2f8bd5a449297dc9d897

note: IIRC there are other MATE apps with arch-dependent files
in /usr/share, so looks like there's more work to do.