Minor clarifications and simplifications in GUI code

Observations in reading GUI code:

• Several using CatalogedType = ... declarations seem to be unused. Append explanatory comment // used in SelectionProperty<Catalog>.
• In RotationsItemCatalog.h, comment "Available types of interference items" seems to contain a copy&paste error. As this is boilerplate code, keep the comment short, and same for all similar classes.
• At several places, typo "than" where it should be "then". Grep for "than" in entire GUI directory please.
• struct UiInfo is declared in identical way in many classes. Why not declare it at global level?
• In FormFactorItemCatalog (and elsewhere), instead of macro name CHECK, which evokes testing, maybe RETURN_IF?
• From name RealItem I cannot guess the meaning. Could refer to floating-point numbers, for instance. What about ExternalDataItem?
• In ItemizeSample.cpp, the stacked template in local function set_PDF2D is very complicated. Is there a way to simplify?