2.1-SNAPSHOT branch created

I would like to wait for 2.0 series to be more stable before with make 2.1-SNAPSHOT the new master branch.

For the time being, I'm using 2.1-SNAPSHOT to avoid messing up the master branch.

I have modified the EntitiyContainer::draw() to draw by the order of layer names and entity IDs, and this is a simple way to allow users to control the drawing order: by specifying priority in layer names.

It seems to be a stable change, if it's stable enough, I would like to cherry-pick the commits to 2.0.3.

A screenshot to show entities drawing by layer name order: