plugins: add zeitgeist plugin from 'gedit' project

Reference: https://bugzilla.gnome.org/show_bug.cgi?id=731799
Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
plugins: fix installing appstream files

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>