Fixed: Updated backgroundColor internal variable handling

Previously, _textFieldBackgroundColor was used as the internal variable, and getter/setter methods were explicitly declared to handle getting/setting this value.

This commit changes the name of the internal variable to match the Cocoa compliant method (backgroundColor/setBackgroundColor). It also declares an @accessor option to auto-generate the getter. The setter is explicitly declared because it needs to mark the object as needing display.

Additionally, the logic for the setting is moved from the older method, setTextFieldBackgroundColor, to the new one, setBackgroundColor, and a comment was made to clarify that the older one was deprecated with a link to this issue.