Fixed Stage#setRoot not calling Actor#setStage.
Fixed Stage#setRoot not calling Actor#setStage.

closes #5476

tommyettinger/libgdx,Zomby2D/libgdx,alex-dorokhov/libgdx,bladecoder/libgdx,alex-dorokhov/libgdx,bladecoder/libgdx,cypherdare/libgdx,libgdx/libgdx,MikkelTAndersen/libgdx,tommyettinger/libgdx,alex-dorokhov/libgdx,MikkelTAndersen/libgdx,MikkelTAndersen/libgdx,Zomby2D/libgdx,tommyettinger/libgdx,Zomby2D/libgdx,Zomby2D/libgdx,NathanSweet/libgdx,cypherdare/libgdx,bladecoder/libgdx,bladecoder/libgdx,alex-dorokhov/libgdx,tommyettinger/libgdx,NathanSweet/libgdx,stinsonga/libgdx,stinsonga/libgdx,alex-dorokhov/libgdx,libgdx/libgdx,NathanSweet/libgdx,tommyettinger/libgdx,libgdx/libgdx,MikkelTAndersen/libgdx,MikkelTAndersen/libgdx,stinsonga/libgdx,cypherdare/libgdx,NathanSweet/libgdx,libgdx/libgdx,stinsonga/libgdx,MikkelTAndersen/libgdx,alex-dorokhov/libgdx,cypherdare/libgdx,cypherdare/libgdx,libgdx/libgdx,alex-dorokhov/libgdx,NathanSweet/libgdx,MikkelTAndersen/libgdx,Zomby2D/libgdx,stinsonga/libgdx,MikkelTAndersen/libgdx,alex-dorokhov/libgdx,bladecoder/libgdx