[scene2d] Added Group#getChild(int) convenience method.
[scene2d] Added Group#getChild(int) convenience method.

It's pretty common to need to `getChildren().get(x)`.

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