Add a way to cusomize the automata

[Lazerbeak12345]

Is your feature request related to a problem? Please describe.
Some of my users have expressed a desire for the ability to have some sort of custom automata setup.

Describe the solution you'd like

• Have someplace (in-app, such as a slide-in/up dialog, or mobile) that lets the user modify a single element called "custom".
• Have a similar dialog, but allow for users to add and remove as many of anything that they would like.

Describe alternatives you've considered
Leaving it as is to avoid further complexity.

Additional context
Other cellular automata tools seem to have this feature.

To make this possible, I'll need to do these:

• Generate the dropdown list options from the same object eventually passed to p.addMultipleElements (or otherwise) #12
• Create a menu to view and customize the current element list #13
• Make a way to "add" an automata. #47