Fixed|libdoomsday|DEDRegister: Finding definitions case-insensitively

The lookup value must be converted to lower case because the values
entered into the lookup dictionary are also lower-cased.
Refactor|libdoomsday|DEDRegister: Case-insensitivity improvements

Now uses a generic lookup method that can be used with any operation
thanks to lambdas. This fixes the handling of lookup values in the
has() method.