Update snake layout logic taking into account small molecules

[ljubica-milovic]

Background
Currently small molecules do not get layouted in snake mode - their position never changes automatically. This might lead to overlap between small molecules and monomers on canvas.
In this ticket, a simplest layouting logic for small molecules is covered, and should be improved later.

Requirements

1. All chains made up from monomers should be layouted like usual at the top of the canvas.

2. Small molecules connected to monomers should be layouted bellow the chains.

3. Small molecules NOT connected to monomers should be layouted bellow the ones who are connected to monomers.