There was a lot of code duplication between the property and ivar editor classes. This fixes that.
- Renamed: FLEXFieldEditorViewController → FLEXVariableEditorViewController
- Renamed: FLEXMutableFieldEditorViewController → FLEXFieldEditorViewController
- Collapsed FLEXPropertyEditorViewController and FLEXIvarEditorViewController into their parent class, FLEXFieldEditorViewController
- Property/ivar editor now takes a FLEXProperty/FLEXIvar
- Property/ivar editor initializer is failable based on editability of property/ivar
- FLEXMethodCallingViewController now takes a FLEXMethod
- Argument input views will now generally allow editing of a nil value