Reimplement add-menus in terms of seesaw

Signed-off-by: Daniel Borchmann <daniel.borchmann@mailbox.tu-dresden.de>
Basic plugin manager now uses new add-menus function

Signed-off-by: Daniel Borchmann <daniel.borchmann@mailbox.tu-dresden.de>
Reimplement menu structure of context editor

Signed-off-by: Daniel Borchmann <daniel.borchmann@mailbox.tu-dresden.de>
Minor code cosmetics

Signed-off-by: Daniel Borchmann <daniel.borchmann@mailbox.tu-dresden.de>