These 2 new actions are meant to be usable on all 3 types of non-raster
(and non-group) layers, i.e. link, text and vector layers, to
respectively rasterize and un-rasterize them.
This will also work with multiple selected layers, and is not specific
to one type of layers.
I also change how gimp_text_layer_discard() used to work, by marking the
text layer as modified instead of actually discarding all text
information. The main consequence of this was that a layer rasterized
this way was forever lost. Now it can actually be revived as a text
layer, not only through the new layers-retrieve action, but also by
trying to edit it with the Text tool, which will trigger the same dialog
as when a text layer had been rasterized by editing it with a paint
tool.
Whereas the label of "layers-rasterize" seem to be quite straightforward
(simply "Rasterize" per discussions in gimp-ux#252), I am really unsure
of the label for "layers-retrieve". Further UX discussions should help
on this front.