Remove filesystem dependency from SaveStl export source
Remove filesystem dependency from SaveStl export source

- Convert to ILibraryContentStream
- Extract UI interactions to SaveStl caller
- Return success result from SaveStl

jlewin/MatterControl,larsbrubaker/MatterControl,mmoening/MatterControl,unlimitedbacon/MatterControl,larsbrubaker/MatterControl,larsbrubaker/MatterControl,jlewin/MatterControl,unlimitedbacon/MatterControl,mmoening/MatterControl,unlimitedbacon/MatterControl,unlimitedbacon/MatterControl,mmoening/MatterControl,jlewin/MatterControl,jlewin/MatterControl,larsbrubaker/MatterControl