Implement a handler for displaying multiple reactions defined in the overlay config
How it works (shorten):
Algo tests if overlay element is bioentity/reaction by certain fields existence in element object
If the overlay element is a reaction, all possible segments (base/reactants etc.) are combined into a polygons array
Then, polygons are rendered as lines starting from left-top point and ending at right-bottom polygon point
It's possible that Left-Top (X1Y1) point will be lower in space than Right-Bottom (X2Y2) point. That's why width/height are defined as an absolute value. However, it does not require additional edge-case coverage. In the case above, the line will be just mirror-reversed and that's an expected behavior
What has been done:
additional handlers for overlay element type of reaction