Arch: Support for extruded curves in IFC export
+ Support .qss file extension for style sheets
+ fixes #0001569: Bounding Box of Mesh::Feature in the Gui does not respect the Placement
Draft: small fix in DXF importer
Draft: Clones are now 2D if made from a 2D object
+ fixes for sketcher
+ implement hexagon sketch
+ optimize creation of hexagonal sketch profile, add icon
+ fix path separator issue in widget factory
+ whitespace improvements
+ abort slot creation in error case
+ change order of tangent and coincident constraints of slot
+ fix issue with default value and unit in InputField
Fixed Datum Filter showing Named non-datum variables
+ change comment
Arch: Misc fixes

* Allow to export subtractions as separate elements in IFC
* Fixed Survey tool
* Removed unused Arch preferences
+ do not close placement dialog if an error occured when pressing OK