Update dialog_base implementation, assets and documentation

locale/schema_en.properties

@@ -27,3 +27,9 @@ dialog/schema/builtin/option = Answer
 dialog/schema/builtin/option_desc = An answer and the label it leads to; with a condition it is shown only while it holds
 dialog/schema/field/option_if = Show if
 dialog/schema/builtin/option_if_desc = For example archetype(rogue) or "stat(cha) >= 5"; empty — always
+dialog/schema/builtin/set = Set variable
+dialog/schema/builtin/set_desc = Store a literal string (name value); name=expression calculates a numeric value.
+dialog/schema/builtin/cset = Calculate variable
+dialog/schema/builtin/cset_desc = Calculate a numeric expression at execution; also accepts name=expression.
+dialog/schema/field/variable = Variable
+dialog/schema/field/expression = Numeric expression