Remember the previuos active tool on selection change
Remember the previuos active tool on selection change

This prevents issues where activating a tool before selecting something
would change to the translate tool.

Fixes #53

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