non-group "layers"

Something which I really missed in SVG is the ability
to work with layers.

Layers should allow to show and hide all objects in
them and it should be possible to change some attribute
in all objects of a layer (for example, the line
color). Alternatively, it should be possible to select
objects by layer - then this can be done manually.

Note that layers are not groups: Objects from different
groups can be in the same layer so they move with the
other objects in the same group but whether they are
visible or not depends on the setting of the layer.

Layers can be used to depth-arrange objects (so when I
move a layer in front of some other layer, the objects
in the different groups must be reordered accordingly).
When a group is in a layer, then all objects in that
group are in the same layer.

I'm not sure if it would be a good idea to allow one
object to be in different groups (it's visible when at
least one of them is visible).

For the implementation: When I did this, I added a
x:layer attribute to the elements plus a x:layer-style
element to the SVG. Then my XSLT script would select
drop all elements of invisible layers and sort the
layers according to the depth attribute of the
x:layer-style elements.

( NOTE: originally in sodipodi bug tracker as bug(s) #
616232 )