DXF export improvements:
* tree view and attribute selection for layer assigment in dialog
* support fill polygons/HATCH
* represent texts as MTEXT instead of TEXT (including font, slant and weight)
* support for RGB colors when there's no exact color match
* use AutoCAD 2000 DXF (R15) instead of R12
* remove R18 test methods

Funded-By: City of Uster
Funded-By: Ville de Morges
Funded-By: SIGE
remove unused ramp