It was not possible anymore for a plug-in to add a menu item to the
local Layers, Classes and Paths dialogs using <Layers>, <Classes> and
<Vectors> at the start of the menu path.
For the first two this was due to an extra length parameter and the
drawables now being inside a GimpObjectArray. To fix that we check
inside the object array for the correct type names.
I'm not sure if Vectors ever worked, but testing shows it only adds
runmode and image parameters. To fix that we do the same as for
ColorMap.