1. minor upgrade of {e t}

In some cases, when you edit a floater dyntext that refers to a concrete 
object attribute directly, the dialog now offers a shorthand to edit the 
value of the attribte.

Note: in many cases the value displayed is coming from the abstract model 
and is a result of the compilation. In such cases it's not possible to 
quickly edit to the source, especially that there may be multiple sources. 
Later on I will make the attr edit button bring up the abstract attribute 
editor where you can figure where it is coming from.

Note: you really shouldn't be doing attribute edits with {e t}. If you 
want to change the name of concrete objects, you should use {a a}. That 
always defaults to changing the "name" attribute. And unlike in many 
other schematics editors, "name" is always the name of the object (we 
don't have "refdes", "netname", "pinname" attributes, just "name").


2. png export

I've moved a lot of low level exporter code from pcb-rnd to librnd. With 
these I could finally enable our png export.

Note: text objects are not yet exported in any of the exporters. This is 
not a bug but a missing feature (pixmap export) that I will implement 
later.


3. export dialog works

I've also moved the export dialog code to librnd so we now have the same 
export GUI in sch-rnd as in pcb-rnd. You can invoke it using {f e} or the 
menu item in the File menu. 


4. export layer visibility control

Most exporters, including ps, eps, png and svg shares an optional 
argument, --layers. This controls which layers are visible on the output. 
It's sort of equivalent to the --cam feature in pcb-rnd, although much 
simpler and less capable, because layers in this context have different 
meaning from pcb-rnd's layers.

This works from both GUI and -x CLI exports.

--layers also replaces the broken --as-shown concept we have in pcb-rnd. 
The --as-shown model is broken in many ways: it's not very well defined 
what exactly it means so even after I tried to unify that in pcb-rnd there 
are tiny differences per exporter. Plus it tried to control way too many 
different things at once (layer visibility, overlays, board side, 
coloring, etc) so it rarely did exactly what you wanted. In sch-rnd we 
start clean, no backward compatibility with decades old bad decisions, so 
I want each export option to do only one thing but do that right.

Syntax documentation for --layers:

http://repo.hu/projects/cschem/user/09_appendix/export_layers.html