Disallow saving values to project for image exports. Fixes #1172.
Disallow saving values to project for image exports. Fixes #1172.

tizzybec/tilemill,MappingKat/tilemill,fxtentacle/tilemill,florianf/tileoven,Teino1978-Corp/Teino1978-Corp-tilemill,nyimbi/tilemill,Zhao-Qi/tilemill,mbrukman/tilemill,florianf/tileoven,florianf/tileoven,tizzybec/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,tizzybec/tilemill,nyimbi/tilemill,tizzybec/tilemill,fxtentacle/tilemill,paulovieira/tilemill-clima,nyimbi/tilemill,MappingKat/tilemill,mbrukman/tilemill,nyimbi/tilemill,fxtentacle/tilemill,mbrukman/tilemill,mbrukman/tilemill,Zhao-Qi/tilemill,paulovieira/tilemill-clima,nyimbi/tilemill,MappingKat/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,tilemill-project/tilemill,tilemill-project/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,Zhao-Qi/tilemill,paulovieira/tilemill-clima,Teino1978-Corp/Teino1978-Corp-tilemill,tizzybec/tilemill,tilemill-project/tilemill,paulovieira/tilemill-clima,Zhao-Qi/tilemill,Zhao-Qi/tilemill,mbrukman/tilemill,fxtentacle/tilemill,paulovieira/tilemill-clima,MappingKat/tilemill,fxtentacle/tilemill,MappingKat/tilemill