Structure GUI improvements

I'm working mainly off the Python version but have been wanting UI customizations and so have been making changes to the C++ side and rebuilding the wheel. One of the things I've had to implement so far is customization for how the Structure GUI adds structures to itself. Right now it adds structures by the typeName, which itself is a hardcoded string in the respective structure class and set as a static variable. It's used all over the place in that context so I ended up adding a defining a new var setting it all in the header for the structure class I wanted to customize the GUI for, then modifying the structureGUI build method to check for this new var and set it in a collapsing header by that instead of its typeName. In the end my solution is kind of hacky and relies on type casting. I would like to request a more official implementation of this feature as well as more customizability for UI elements, etc. as built in.

Start with the Structure GUI build method and the relevant structure class headers described in the issue; no specific files or tests are named. Define the scope of an official structure-customization mechanism and built-in UI customization, then confirm the resulting API avoids the current type-casting workaround.