Add other boolean modes to the Boolean Operation node

[devviktoria]

On discord boldline suggested to implement the following feature:
Would you consider adding a "merge" option to the boolean tools? Specifically one that functions like in Illustrator and in Vectorstyler? Essentially, if the two objects are the same color, it will merge them together, if they are different colors, it will cookie-cutter them but leave both pieces there. And also @Keavon suggested to add the other boolean modes that can be constructed out of the existing boolean ops algorithm from the Linesweeper library that would be expected in other vector graphics apps.

So far Graphite implemented: Union, Subtract front, Subtract back, Intersect, Difference (this one is equivalent to Illustrator's Exclude, VectorStyler's Remove Overlaps, Inkscape's Exclusion. But maybe a better name can be used for this?)

My proposal is to add the following operators to the Boolean Operation Node:

• Merge: Apply union for all the objects that share the same fill property, removes strokes from all objects (Equivalent of Illustrator's Merge, VectorStyler's Merge functionality)
• Divide/Fracture: Calculates all the overlapping and non overlapping parts (Equivalent of Illustrator's Divide, VectorStyler's Divide, Inkscape's Fracture)
• Trim: Subtracts top objects from the objects below them and keeps the remainder of the operation and also the top objects, removes stroke from all objects (Equivalent of Illustrator's Trim, VectorStyler's Trim functionality)

Reference material:
How to Use the Pathfinder Panel in Adobe Illustrator (Full Overview)
https://www.youtube.com/watch?v=mkNnyDEVKUY
The Pathfinder Tool Explained // Adobe Illustrator Tutorial
https://www.youtube.com/watch?v=tJLMnaP3VyM
https://helpx.adobe.com/illustrator/desktop/manage-objects/reshape-transform-objects/pathfinder-panel-overview.html

Boolean Operations Reference Guide
https://vectorstyler.com/forum/topic/1758/boolean-operations-reference-guide/2

All Boolean Path Operations in Inkscape Explained (with Examples)
https://www.youtube.com/watch?v=Xr8HqSwqn2E
https://inkscape-manuals.readthedocs.io/en/latest/boolean-operations.html

[devviktoria]

I think I am going to start with the Merge functionality since this was requested on discord.
These operations are going to give back a list of objects. So what I am thinking that this part can be achieved with the behavior of the Radial repeat node for example, and to extract one element one can use the Index Elements node.
I am going to explore if the existing boolean node can be used for this or perhaps a different kind of boolean node is needed for this.
[EDIT:] So I looked at the boolean node and it seems that there is a TODO for making it work with multiple items, I think that is not a problem since the boolean operations already returning a table.
However I am looking into the linesweeper library. I started to work on the Divide/Fracture operation instead, but first I decided to figure out if that is possible to do with the linesweeper library. And if that is possible I will integrate the algorithm into graphite. But it seems that this is not trivial to figure out so it is going to take a bit of time.