Makes the openfile button blue/active as long as there is no file loaded.
Makes the openfile button blue/active as long as there is no file loaded.

This is to make the UI more intuitive.

Contributes to #43

totalretribution/Cura,hmflash/Cura,ynotstartups/Wanhao,senttech/Cura,ad1217/Cura,totalretribution/Cura,markwal/Cura,bq/Ultimaker-Cura,DeskboxBrazil/Cura,quillford/Cura,ynotstartups/Wanhao,lo0ol/Ultimaker-Cura,bq/Ultimaker-Cura,fieldOfView/Cura,fxtentacle/Cura,DeskboxBrazil/Cura,quillford/Cura,fxtentacle/Cura,fieldOfView/Cura,lo0ol/Ultimaker-Cura,Curahelper/Cura,markwal/Cura,senttech/Cura,ad1217/Cura,hmflash/Cura,Curahelper/Cura