Added Stage#setRoot.
Added Stage#setRoot.

closes #3658

josephknight/libgdx,libgdx/libgdx,fwolff/libgdx,josephknight/libgdx,libgdx/libgdx,codepoke/libgdx,js78/libgdx,NathanSweet/libgdx,realitix/libgdx,FredGithub/libgdx,tommyettinger/libgdx,FredGithub/libgdx,alex-dorokhov/libgdx,ttencate/libgdx,cypherdare/libgdx,sarkanyi/libgdx,czyzby/libgdx,samskivert/libgdx,czyzby/libgdx,josephknight/libgdx,ttencate/libgdx,fwolff/libgdx,ttencate/libgdx,tommyettinger/libgdx,NathanSweet/libgdx,sarkanyi/libgdx,samskivert/libgdx,alex-dorokhov/libgdx,hyvas/libgdx,MikkelTAndersen/libgdx,kotcrab/libgdx,codepoke/libgdx,realitix/libgdx,stinsonga/libgdx,BlueRiverInteractive/libgdx,bladecoder/libgdx,hyvas/libgdx,js78/libgdx,codepoke/libgdx,cypherdare/libgdx,MovingBlocks/libgdx,fwolff/libgdx,bladecoder/libgdx,ttencate/libgdx,MovingBlocks/libgdx,alex-dorokhov/libgdx,realitix/libgdx,fwolff/libgdx,BlueRiverInteractive/libgdx,BlueRiverInteractive/libgdx,hyvas/libgdx,MikkelTAndersen/libgdx,codepoke/libgdx,BlueRiverInteractive/libgdx,MikkelTAndersen/libgdx,realitix/libgdx,FredGithub/libgdx,tommyettinger/libgdx,kotcrab/libgdx,realitix/libgdx,Zomby2D/libgdx,samskivert/libgdx,czyzby/libgdx,sarkanyi/libgdx,alex-dorokhov/libgdx,ttencate/libgdx,hyvas/libgdx,Zomby2D/libgdx,Zomby2D/libgdx,codepoke/libgdx,alex-dorokhov/libgdx,MikkelTAndersen/libgdx,alex-dorokhov/libgdx,MikkelTAndersen/libgdx,bladecoder/libgdx,alex-dorokhov/libgdx,tommyettinger/libgdx,Zomby2D/libgdx,kotcrab/libgdx,fwolff/libgdx,realitix/libgdx,czyzby/libgdx,js78/libgdx,Zomby2D/libgdx,fwolff/libgdx,sarkanyi/libgdx,czyzby/libgdx,MovingBlocks/libgdx,FredGithub/libgdx,codepoke/libgdx,cypherdare/libgdx,libgdx/libgdx,cypherdare/libgdx,realitix/libgdx,MikkelTAndersen/libgdx,stinsonga/libgdx,codepoke/libgdx,cypherdare/libgdx,sarkanyi/libgdx,MovingBlocks/libgdx,hyvas/libgdx,bladecoder/libgdx,fwolff/libgdx,josephknight/libgdx,NathanSweet/libgdx,libgdx/libgdx,hyvas/libgdx,josephknight/libgdx,codepoke/libgdx,sarkanyi/libgdx,ttencate/libgdx,js78/libgdx,BlueRiverInteractive/libgdx,MovingBlocks/libgdx,ttencate/libgdx,samskivert/libgdx,FredGithub/libgdx,realitix/libgdx,NathanSweet/libgdx,kotcrab/libgdx,hyvas/libgdx,czyzby/libgdx,MovingBlocks/libgdx,NathanSweet/libgdx,BlueRiverInteractive/libgdx,czyzby/libgdx,MikkelTAndersen/libgdx,MovingBlocks/libgdx,bladecoder/libgdx,samskivert/libgdx,js78/libgdx,kotcrab/libgdx,stinsonga/libgdx,hyvas/libgdx,samskivert/libgdx,js78/libgdx,FredGithub/libgdx,FredGithub/libgdx,kotcrab/libgdx,fwolff/libgdx,tommyettinger/libgdx,js78/libgdx,samskivert/libgdx,kotcrab/libgdx,kotcrab/libgdx,josephknight/libgdx,ttencate/libgdx,js78/libgdx,libgdx/libgdx,MovingBlocks/libgdx,alex-dorokhov/libgdx,BlueRiverInteractive/libgdx,FredGithub/libgdx,stinsonga/libgdx,sarkanyi/libgdx,czyzby/libgdx,stinsonga/libgdx,sarkanyi/libgdx,samskivert/libgdx,josephknight/libgdx,BlueRiverInteractive/libgdx,MikkelTAndersen/libgdx,josephknight/libgdx