add Affine2 matrix, optimize scene2d group transform
GWT compatibility; name change
Added test cases, removed confusing getScale/getRotation

Reducing matrix to original components depends on concatenation order.
Something like decomposeRotScl() & decomposeSclRot() could be added
instead, but does anyone need it?
Merge pull request #2330 from vmilea/affine

add Affine2 matrix, optimize scene2d group transform