In addition to geometry filtered layers for each geometry
type in a mixed-geometry type layer adds a table
layer with all attributes.
Tehre is no perfetc solution here, if the layer is created
with a filter on geometrytype, the attributes are not shown
(this is probably a pre-existing bug), if the layer is added
as is, only the first geometry type is drawn.
With this implementation at least all data (attributes and
geometries) are accessible in some way.
Note that geopackage layers added by DB Manager do not show
all geometries of a mixed type layer.