Add GraphState to Graph Editor (#2762)

This changelist introduces a `GraphState` struct to encapsulate the UI state for a graph level in the MaterialX Graph Editor.  The following improvements are included:

- Consolidate all variables for the UI state of a single graph level into a `GraphState` struct.
- Simplify nodegraph navigation by merging parallel stacks into a single `_parentStates` vector.
- Split the `_initial` flag into `_needsLayout` and `_needsNavigation` for clarity.
- Fix edge cases in `UiPin::deleteConnection`, where a loop break and reset of the `_connected` flag were needed for completeness.

Author: jstone-lucasfilm