Replace "Scroll" check box with a combo box (fixes #3748)

Replaces the "Scroll" check box in Book records with a "Book Type" combo box.

Related issue:
- Fixes #3748: OpenMW-CS: Replace "Scroll" check box in Book records with "Book Type" combo box. (https://bugs.openmw.org/issues/3748)

Tests:
The changes were successfully tested in OpenMW-CS by manipulating several Book records. Please note that the actual logic behind this entry is not implemented yet: Books which are of type "Scroll" can have an enchantment attached, normal books ("Book") cannot.

apps/opencs/model/world/refidadapterimp.cpp

@@ -313,7 +313,7 @@ QVariant CSMWorld::BookRefIdAdapter::getData (const RefIdColumn *column,
         data.getRecord (RefIdData::LocalIndex (index, UniversalId::Type_Book)));
 
     if (column==mBookType)
-        return record.get().mData.mIsScroll!=0;
+        return record.get().mData.mIsScroll;
 
     if (column==mSkill)
         return record.get().mData.mSkillId;