* Added conversion extension methods for Vector4, Quaternion to each other.

* Modified GenericPropertyDrawer to include edge case checks for both layout and non layout modes when the type of value being displayed is a Vector2/3/4 and Quaternion so that they can be displayed in the proper format for Variables, Collections, and References.
* Modified the CollectionEditor to include an edge case for Vector4, Quaternion to display them on a single line rather than use the reported property height which covers several lines.
* Modified BaseReferenceDrawer to draw constant values using GenericPropertyDrawer methods; this has the benefit of keeping existing functionality for constants, plus covering edge cases for Vector2/3/4 and Quaternions.

* Removed redundant call to set rect height as it the height for an element in a reorderable list is set by its `elementHeightCallback`, which in this case is `GetElementHeight`. Removed padding `GetElementHeight` so that a single line item does not appear stretched.

* This is content added to make it easier to test this PR in the editor, it can be discarded after reviewing.