The idea is simple: we insert a pre-formatted table (with a given border
style currently, but other attributes are possible as well) via file-insert
and scale it then to the requested size.
We need three sizes (1x1, 1x2 and 1x3) to generate all sizes properly (due
to border specifications).
Currently, these styles can only be accessed via lfun tabular-style-insert
and the Tabular Create dialog.
My plan is to add a buffer param to set a default style (#9901) which then
also is respected by the toolbar button and probably a layout tag to let
classes specify a default style (#8360).
